Aesthetic – Relating to the creation and appreciation of beauty. The aesthetic appeal of an aquarium refers to how well-decorated and pleasing it is to the eye of the hobbyist.
Aged water – Tap water that's been left in an open bucket or container for 24 hours or more so the chlorine will evaporate out of it. This method is an alternative to using dechlorinating solutions.
Aggressive – A term used to describe species of fish which tend to become violent toward other fish. Males of aggressive species typically cannot be kept in the same tank with other males of the same species. Many of the larger species of fish including Cichlids are considered aggressive.
Acclimation – The process of slowly introducing new fish to the aquarium, giving them time to adjust to the new water conditions.
Acidic – A term used to describe a solution having a pH value less than 7.0.
Acriflavine – A type of topical antiseptic often used to treat external fungal infections in aquarium fish.
Acrylic – Or acrylic glass. A generic term for a lightweight, shatter-resistant plastic often used as an alternative to glass in building aquariums.
Actinic – A type of aquarium light from the blue end of the spectrum. This type of light is able to penetrate depths of more than 30 feet and is essential for the healthy growth of corals and other photosynthetic organisms.
Activated carbon – Utilized as filter media in various types of aquarium filters. This substance is derived from charcoal and is designed to absorb dissolved wastes and toxins from aquarium water through the process of chemical filtration.
Adipose fin – The small fin on the backs of aquarium fish, located between the dorsal and caudal fins.
Aeration – The process of circulating air through aquarium water. This is accomplished through the use of aquarium filters or air pumps.
Aerobic – A term meaning “requiring air.” Aerobic organisms require oxygenated environments in order to grow and thrive.
Air pump – Injects air into aquarium water - often a component of aquarium filters.
Air stone – A block of porous stone through which air is forced by an air pump. As the air passes through the stone, it is diffused in a stream of micro-bubbles which oxygenate tank water.
Algae – A group of autotrophic, photosynthetic organisms that grow in the aquarium under certain conditions. These organisms require nutrients, light and carbon dioxide in order to grow.
Algae bloom – A sudden increase in algae growth that may result in a green or brown discoloration of tank water. Algae blooms are often a result of water quality issues; i.e. an excess of nutrients, and are common in new, un-cycled tanks.
Algae scrubber – A tool used to remove algae build-up from tank walls and decorations. These tools typically consist of a sponge or scrubber pad affixed to a long handle.
Alkaline – A term used to describe a solution having a pH value greater than 7.0.
Alkalinity – Also called “buffering capacity” – the measurement of the ability of aqurium water to resist changes in pH. Typically measured in carbonate alkalinity or carbonate hardness (kH).
Ammonia – A compound of nitrogen and hydrogen known by the formula NH3. This substance is produced as gas in the aquarium during the breakdown of organic waste by nitrifying bacteria. Ammonia is toxic to aquarium fish and can cause ammonia poisoning. Ammonia levels in the aquarium should always be kept at 0ppm (parts per million).
Amoeboid – One of four types of parasite. Amoeboids are shapeless, single-celled organisms which propel themselves through the use of pseudopods, or false feet.
Anabantid – Or labyrinth fish. The family anabantoidei is a group of fishes possessing a lung-like organ called a labyrinth that enables the fish to breathe air.
Anaerobic – A term meaning “without oxygen.” Anaerobic organisms, like some bacteria, do not require oxygen for growth and may in fact die if oxygen is present.
Anal fin – The single fin located on the underside of the fish in front of the caudal fin.
Aquascape – The arrangement of decorative plants, rocks and other items in an aquarium. Aquascaping is the craft of arranging these items.
Aquarist – Or fishkeeper. A person engaged in the aquarium hobby.
Aragonite – A carbonate mineral, the most common natural form of calcium carbonate (CaCO3). This substance forms as the exoskeleton of corals and can be found in some types of marine sand and substrates. When used in substrate, aragonite can help harden water and raise pH.
Artemia – The scientific name for brine shrimp.
Auxiliary fan – A type of cooling fan often used in conjunction with heat-producing aquarium lighting systems such as metal halide lights.
Ballast – A specialized power source required by certain types of aquarium lighting including fluorescent and metal halide lights. A ballast serves to ground the unstable gases in these bulbs and may or may not be built directly into the fixture.
Barbels – Whisker-like appendages protruding from the mouth of most catfish. These sensitive appendages are used when searching for food.
Base – A compound that, when added to a solution, makes it more alkaline. If aquarium water contains more bases than acids, it will be alkaline. If the water contains more acids than bases, it will be acidic.
Benthic – Or benthic zone. Referring to an ecological region in the lowest level of a body of water, including the substrate and the area below. Organisms living in this area are called benthos.
Bioload – Also referred to as “biological load” or “bio-load”. This basically refers to the amount of life (fish, invertebrates, etc.) in the aquarium. A bio-load that is too high will cause a reduction in oxygen levels available to tank inhabitants. If you notice tank inhabitants hanging out near the surface gasping for air, this could be a sign of a high bio-load. Prevent this by not overstocking your tank.
Biological filtration – The process through which beneficial bacteria breakdown harmful ammonia and nitrites, converting them into nitrates. This process is naturally occurring, but can be further facilitated through the use of biological filter components such as bio-wheel attachments for hang-on-the-back filters and ceramic media inside canister filters.
Blackwater – Referring to a body of water, typically a river, stained by high plant content. Rivers like the Amazon are said to be blackwater rivers because they flow through richly vegetated areas and the plants in and around the river leak tannins into the river, staining it brown and softening the water.
Blanch – To boil vegetables for a brief period of time. Aquarium fish are more likely to accept fresh vegetables if they have been blanched first.
Bleaching – The process through which corals expel their zooxanthellae, thereby losing their pigmentation and turning white or pale. This process may occur when the corals are under stress.
Bloodworm – Mosquito larvae, often fed to aquarium fish in live, frozen and freeze-dried form. Bloodworms are a good source of protein for aquarium fish and can be used to encourage fussy fish to eat.
Bottom feeder – Species of fish, invertebrates and crustaceans that tend to live and feed in the bottom level of the tank. These organisms are typically scavengers, feeding on detritus and sinking foods.
Brackish – A term used to describe a mix of fresh and salt water. This type of environment occurs naturally in estuaries at the mouths of rivers, in mangrove swamps and in certain lakes and small oceans. Some fish and invertebrates, such as fiddler crabs, prefer a brackish environment.
Brine shrimp – Also called Artemia. A type of aquatic crustacean used as food for aquarium fish. These organisms are high in fat and, when newly hatched, are the perfect food for fry.
Bubble nest – A nest constructed of bubbles on the surface of the aquarium, held together by saliva. These nests are often built by the males of Anabantoid species, such as Gouramis and Bettas, and are designed to hold the eggs after spawning has occurred.
Bubble wand – A decorative device powered by an air pump designed to create a “curtain” of bubbles along the length of the wand. Some bubble wands can be bent or shaped around tank objects.
Buffer – As a verb, buffer is synonymous with “stabilize” – it refers to the ability of aquarium water to maintain a steady pH despite small changes in water conditions. Buffer may also refer to a buffering solution which is a substance added to aquarium water to raise the alkalinity or change the pH.
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Calcium carbonate – A chemical compound (CaCo3) naturally found in sea shells and the skeletons of corals.
Canister filter – A type of external filtration system which utilizes mechanical, chemical and biological filtration. These filters consist of a tube or canister filled with filter media through which aquarium water is pumped then returned to the tank via an outlet tube. Canister filters are a popular choice for freshwater aquariums.
Carbonate hardness – Also called carbonate alkalinity. A measure of the alkalinity of tank water based on the concentration of carbonate and bicarbonate ions. Measured in parts per million (ppm) or degrees KH (dKH).
Carnivore – A type of organism which derives most of its nutrients from meat-based foods.
Catfish – A large group of freshwater fishes named for their barbels which resemble the whiskers of a cat. Catfish do not have scales, but some species have armor-like plates covering their bodies.
Caudal fin – Tail fin. The single fin located at the end of the fish’s body.
Caudal peduncle – The narrow portion of a fish’s body to which the caudal fin is attached.
Cavity spawner – A type of aquarium fish that lays its eggs in a cave or other cavity within the tank. These fish often exhibit advanced care of their young, defending and cleaning the spawning site until the eggs hatch and caring for the fry afterward.
Characin – A fish belonging to the Characiforme order. Originally, all Characins were groups in the single family Characidae but now the order contains 18 different families. The most recognizable species included in this order are Piranhas and Tetras.
Chemical filtration – A method of filtration through which dissolved wastes and toxins are removed from tank water. Aquarium filters that utilize chemical filtration often require the use of activated carbon or ion-exchanging resin as filter media.
Chiller – A device used to cool aquarium water. These devices are often used to counteract heat-producing lighting systems such as metal halide lights.
Chloramine – A substance often added to tap water to kill bacteria, making it safe for human consumption. This substance is harmful to fish and will kill beneficial bacteria in the tank. Chloramine does not evaporate – water conditioners are required to remove this substance.
Chlorine – A chemical element often used to treat tap water. Though chlorine makes tap water safe for human consumption, it is toxic to aquarium fish and invertebrates. Chlorine can be removed from tap water through evaporation or through the use of water conditioners.
Ciliates – One of four types of parasite. Ciliates propel themselves through the use of myriad short appendages called cilia.
Circulation – The healthy movement of tank water, aiding in the distribution of oxygen and trace elements.
Colony – A significant population of organisms. This term is often used to refer to bacteria in the aquarium, i.e. the colony of beneficial bacteria which plays an essential role in maintaining the nitrogen cycle.
Community fish – Peaceful species of fish that get along well with many other species. Many species of community fish are best kept in groups with others of their own species.
Community tank – A fish tank populated with multiple different species which are all compatible with one another.
Compact fluorescent lights – A type of fluorescent light that comes in smaller, more compact bulbs. These bulbs can put out twice as much light as standard fluorescent bulbs, making them both convenient and energy efficient.
Compatibility – Referring to the ability of one species of fish to get along with another species in the same tank.
Conditioner – A liquid additive used to remove chlorine and heavy metals from tap water in order to make it safe for aquarium fish. Some conditioners contain additional ingredients to boost the slime coat of fish to reduce stress.
Conditioning – The process of preparing aquarium fish for breeding by feeding them plenty of nutritious live and frozen foods. For most species, conditioning should last at least 7 days and males and females should be conditioned separately.
Conspecific – Belonging to the same species.
Copper sulfate – A chemical compound having the chemical formula CuSO4. This substance is often used to treat parasite and fungal infections in fish and can also be used to kill aquarium snails. Because copper can be very toxic to fish, great care must be taken in administering the proper dosage.
Coralline algae – A type of algae, often red or pink in color, that encrusts tank walls and live rock in marine aquariums. These algae are an important food source for certain fish and marine invertebrates.
Crustacean – A type of hard-shelled, aquatic invertebrate such as crabs, lobsters, shrimp and crayfish.
Cyanobacteria – Also known as blue-green algae. A type of bacteria that tends to grow rapidly in tanks with poor water quality or excess nutrients.
Cycling – The process through which the nitrogen cycle is established in a new tank. This process can take up to two months unless the aquarist takes steps to speed up the process.
Cyprinid – A fish belonging to the family Cyprinidae. This family of fishes includes Carps, Minnows, and Barbs.
Cyst – A closed sac containing air, fluid or other material. Many parasite reproduce by forming cysts that attach to the bodies of fish or grow in the substrate of the aquarium. When these cysts burst, the parasites are dispersed throughout the aquarium.
Daphnia – Small aquatic crustaceans often called water fleas. These organisms are a popular food for aquarium fish and can easily be raised at home or purchased online.
Deionization – A purification process for tap water by which impurities such as trace elements are removed. Deionizers are often paired with reverse osmosis filters to keep aquarium water pure.
Denitrification – A process facilitated by anaerobic bacteria through which nitrates are converted into nitrogen gas. This gas then evaporates out of the aquarium.
Detritus – Organic wastes such as dead organisms, decaying plant matter, uneaten fish food and fecal material. These substances tend to accumulate in the substrate of the tank, especially in areas of low water flow.
Diatom – A type of unicellular algae that typically forms brown clumps or films on tank walls and decorative items.
Depigmentation – A loss of color.
Diagnosis – The identification of the cause of a disease in aquarium fish. A diagnosis is typically made by examining the symptoms exhibited by infected fish.
Display tank – An aquarium used to house and display aquarium fish.
Distilled water – Water that has been purified through the process of boiling and then condensing the steam. Distilled water can be used to soften aquarium water, but is not recommended for use in large quantities in the aquarium because it is devoid of essential minerals.
Diurnal – Referring to organisms that are most active during the day, sleeping at night.
Dorsal fin – The fin located along the backs of aquarium fish used for stabilization during swimming. Some species of fish, like Clownfish, have two dorsal fins, one behind the other.
Doser – Or dosing pump. A device used to add supplements or trace elements to aquarium water in controlled amounts. These devices can also be used to replace aquarium water as it evaporates from the tank to keep water levels stable.
Eco-complete – A type of high-quality substrate designed especially for planted tanks. This type of substrate contains live heterotrophic bacteria as well as a variety of essential nutrients to help plants grow and thrive. One of the best options for substrate in planted tanks.
Ectoparasite − Parasite that lives on the external surfaces of the host. On aquarium fish ectoparasites can be found on the gills, mouth, fins, and skin.
Egg burier – A type of fish that buries its eggs in mud or substrate. Many of these fish are annual species like killifish which lay their eggs before dying as the water in their habitat dries up. The eggs lie dormant until the rainy season returns and stimulates their hatching.
Egg depositer – A type of aquarium fish that deposits adhesive eggs on a particular surface in the tank, i.e. the tank glass, rocks, driftwood or plants. The eggs of these fish are typically larger in size than those of egg scatterers but may be fewer in number.
Egg layer – A type of aquarium fish that lays eggs rather than producing live young. Egg-laying species of fish may reproduce through either interior or external fertilization, depending on the species.
Egg scatterer – A type of aquarium fish that scatters adhesive or non-adhesive eggs among the substrate and plants in the aquarium. These fish typically do not display any parental care and the brood size is often very large. The eggs of these fish tend to hatch quickly.
Electrolyte – A substance containing free ions - often solutions containing acids, bases or salts - which make that substance electrically conducive. Electrolytes that are essential in the aquarium include calcium, magnesium and sodium.
Endemic – Unique to a certain geographical location. An organism that is endemic to a particular region is not found anywhere else on Earth.
Endoparasite − Parasite that lives on the inside of the host - normally inhabiting the blood, tissues, and/or organs.
Epsom salt – A chemical compound having the name magnesium sulfate and the chemical formula MgSO4. Epsom salt is often used to create salt baths for treating aquarium fish diseases and it can also be used to raise the water hardness or pH in a freshwater tank.
External infection – An infection or disease that affects the exterior tissues of infected aquarium fish, namely the eyes, mouth, gills, skin and fins. External infections are much easier to diagnose than internal infections because the symptoms are visible.
Family – A taxonomic rank used in classifying living organisms. The family of any given organism falls between the order and genus.
Feeding cone – A plastic cone designed for use in offering aquarium fish live foods. The cone is often attached to a plastic ring which keeps the cone afloat while the cone itself has slits or holes in the side to provide fish with access to the live food inside.
Filter – A device used to clean and circulate aquarium water. Aquarium filters may utilize any combination of three different types of filtration: mechanical, chemical and biological.
Filter feeder – An organism which feeds by filtering food particles from water and substrate.
Filter media – The substance which does the work of actually filtering aquarium water, often contained within a filter cartridge. Aquarium water is pumped through the filter media which catches solid debris and filters out dissolved wastes and toxins.
Fish only (FO) – A tank housing fish only – no corals, live rocks or live aquarium plants.
Flagellate – One of four types of parasite. Flagellates propel themselves through the use of flagella, whip-like appendages which typically appear in pairs.
Fluidized bed – A type of biological filtration in which tank water is forced through a cylinder containing live nitrifying bacteria. These bacteria remove waste from the water before it is returned to the tank.
Fluorescent lights – A type of inexpensive aquarium lighting that utilizes tube-like bulbs. Fluorescent light bulbs come in a variety of sizes and wattages and they can be used for both freshwater and marine aquarium setups.
Fluorite – A lightweight, porous clay gravel substrate often used in planted tanks. This substrate provides plants with valuable nutrients they need to grow and thrive.
Foam fractionation – A filtration method utilized by protein skimmers. In this process, protein molecules are separated from the water using rising columns of foam. Protein skimmers are not necessary for use in freshwater aquariums.
Formalin – A commercial solution of formaldehyde in water, often used as a treatment for bacterial and parasite infections in aquarium fish.
FOWLR – Fish Only With Live Rock. A common configuration of saltwater tanks in which fish are kept with live rocks – no coral is used in this configuration.
Fragging – A process used to propagate corals. In this process, fragments or “frags” of coral are broken off of healthy corals and transplanted to begin a new coral growth.
Freeze-drying – Or freeze-dried. The process of dehydration which is used to preserve certain kinds of fish foods. Though freeze-drying makes these foods easier to store and more convenient to use, the process leaches a significant percentage of the nutrients from the food.
Fry – Newly hatched fish.
Fungus – A type of eukaryotic organism. These organisms are responsible for a variety of aquarium fish diseases including gill rot, cotton wool disease and egg fungus.
Gang valve – A valve often used in air pumps that makes it possible to split the airline output to power multiple devices.
Garlic supplements – A type of dietary supplement derived from garlic oil. These supplements are said to help treat parasitic fish diseases and may also act as an appetite stimulant.
Gas exchange – The process by which gases contained in aquarium water escape and new gases, like oxygen, are dissolved into the water. This process generally occurs at the surface of the tank.
Genus – A taxonomic classification level that falls between the family name and species name.
gH – General hardness - also called dgH or °gH (degrees of general hardness). GH is a measure of the concentration of dissolved ions in tank water.
Gills – The respiratory organ used by fish to extract oxygen from water. Gills typically consist of membranous tissues located on either side of the body behind the eyes of the fish.
Gonopodium – The anal fin in the males of some species of fish which has been modified into a genital organ. This organ is often tube-like in shape, used to impregnate female fishes.
GPH – Gallons per hour. A measurement used to describe the flow rate of aquarium filters.
Gravel vacuum – A tool consisting of a length of plastic hose connected to a plastic tube used to siphon dirty aquarium water and clean substrate.
Gravid spot – The area of dark coloration that develops behind the stomach of females during pregnancy in many livebearers.
Grow-out tank – A separate tank used in raising fry to maturity. Once the fry of certain species reach a particular size, they may be removed to separate grow-out tanks which provide them with adequate space to grow and safety from the parents, or other fish, who may eat them.
Guaranteed analysis – The section on a fish food label which outlines the percentage of crude protein, fat, fiber, moisture, vitamins and minerals in the food.
Gut Loading − The process of feeding live insects, such as bloodworms or white worms, nutritious foods in the hopes of passing those nutrients on to the fish in your tank when they eat the worms. Feeder fish, such as Goldfish, can be gut loaded with nutrients as well as a way to provide additional nutrition to carnivorous fish like Oscars and Arowanas.
Halogen lights – A type of incandescent aquarium lighting which produces light from the yellow end of the spectrum. These lights are not recommended for reef aquariums because they do not produce the right spectrum of light. Yellow-spectrum lighting may encourage algae growth.
Hard water – Aquarium water having a high mineral content. Measured on the scale of general hardness, hard water typically measures between 121 and 180 in mg/L and between 6.78 and 10.08 in degrees of general hardness (°dH).
Head height – The distance between an air pump or filter and the highest point to which the water must be pumped. Pressurized pumps are able to accommodate the greatest head heights. Increasing the head height of a pump or filter will decrease the GPH output.
Heater – A device used to stabilize and change the water temperature in an aquarium. These devices may hang on the back of the tank or they may be entirely submerged for greater heat distribution. Some heaters are programmed to heat water to a certain temperature while others have external temperature controls.
Herbivore – An organism that derives most of its nutrition from plant-based foods.
Hitchhiker – An organism that enters the tank without your knowledge, usually through the introduction of live plants, live rock and new tank water. Aquarium snails are an example of a hitchhiker.
HOB - Acronym for "Hang On Back". See Hang-On Filters.
Hang-On Filters – Also known as Hang on the Back or HOB for short. Refers to a type of external aquarium filter that hangs on the back of the aquarium and sucks up water through an intake tube. One of the most basic filter options for the aquarium. Hang-on filters are relatively inexpensive, very efficient and very easy to maintain.
Hybrid – Offspring resulting from crossbreeding two different subspecies or breeds within a species of aquarium fish.
Hydrometer – A device used to measure the density of tank water in order to determine the specific gravity, or salt content. These devices are temperature dependent – the reading may be different at different water temperatures.
Ich – Ichthyophthirius multifiliis. Also called white spot disease or freshwater ich. One of the most common diseases affecting aquarium fish, Ich is caused by a protozoa parasite which forms white nodules on the skin, gills and fins of aquarium fish. This disease is highly contagious but easily treated.
Immersible – A term used to describe aquarium equipment that can be immersed in water, but not totally submerged. Parts of the equipment must remain above the water level while others may run underwater.
Impeller – A rotor inside many aquarium pumps and filters that functions like a propeller, causing water to flow through the device.
Incandescent light – A type of inexpensive lighting that produces light from the yellow end of the spectrum. Due to the spectrum of the light produced, and because these lights produce heat and encourage algae growth, they are not recommended for aquarium use.
Infusoria – Tiny aquatic creatures often used as food for newly hatched fry.
Internal filter – A type of aquarium filter which operates entirely within the tank itself, being either partially or fully submerged. Examples of internal filters include sponge filters and undergravel filters, both of which provide biological filtration.
Internal infection – See Systemic infection.
Invertebrate – An animal that does not have a backbone. Examples of aquarium invertebrates include crustaceans, corals, snails, shrimp and crabs.
Iodine – A chemical element found naturally in sea water in trace amounts. This substance is necessary in small quantities to maintain the health of some reef invertebrates including clams and corals.
Kalkwasser – A calcium hydroxide solution (CA(OH)2) which can be added to the aquarium in controlled doses to help maintain pH, alkalinity and calcium levels. This substance is often used in reef tanks to replace the calcium used by corals and clams in building their skeletons. Aquarists should exercise extreme caution when using this substance because an overdose can lead to extreme spikes in pH and alkalinity which can be fatal for fish. Not used in freshwater aquariums.
KH – See carbonate hardness.
Krill – A shrimp-like marine crustacean often used as food for aquarium fish in frozen and freeze-dried form.
Labyrinth – A lung-like organ which enables certain species of aquarium fish to breathe air. This organ allows fish to take oxygen directly from the air which means fish possessing this organ are capable of surviving out of the water for short periods of time.
Larvae – The juvenile form of fish and invertebrates – the first stage of development after hatching.
Lateral line – A line of perforated scales running horizontally along the flanks of fish which compose a pressure-sensitive nervous system. The sensory ability derived from the lateral line plays an important role in schooling behavior, allowing fish to detect vibrations in the water caused by other fish and by their own movements.
Lateral line erosion – Also called “hole in the head disease.” This disease affects both freshwater and marine fishes, causing the development of pits and lesions on the body. The causes of this disease are up for debate – the parasite Hexamita was once believed to be the main cause but this is no longer thought to be correct.
Laterite – A type of porous soil recommended as a supplementary substrate for planted tanks. This substrate is rich in iron oxide and helps to facilitate healthy cation exchange.
LED – Light Emitting Diode. A type of aquarium lighting which uses tiny bulbs to produce bright, energy efficient light. Some LED lighting systems incorporate a lunar light function designed to mimic the moonlight preferred by nocturnal tank inhabitants.
Lesion – An abnormality in the tissue of aquarium fish caused by disease or injury. Lesions are a common symptom of parasite infections.
Light meter – A device used to measure the intensity of light. These devices are particularly useful in reef aquariums for measuring the amount of light corals in the tank receive.
Lipids – Fats. An essential part of a healthy diet for aquarium fish, lipids are an important energy source.
Live-bearer – A type of fish that gives birth to live young. Examples of live-bearers include Guppies, Mollies, Platies and Swordtails.
Live rock – A type of rock commonly used in marine and reef aquariums which may be home to various life forms including coralline algae and microscopic marine organisms. The rocks themselves are not alive but are formed from the skeletons of once-living organisms like corals.
Lumen – A measurement of the amount of light emitted by a light bulb.
Lunar lights – A function of many LED lighting systems designed to mimic natural moonlight. This type of lighting system is beneficial for nocturnal aquarium inhabitants.
Macroalgae – Large-celled, photosynthetic algae which vary in color from red to green to brown. Examples of macroalgae include Caulerpa, Maiden’s Hair and Halimeda.
Malachite green – An organic compound often used to treat parasitic, bacterial and fungal infections in aquarium fish. This treatment is particularly effective against freshwater Ich.
Malnutrition – A condition in aquarium fish resulting from an unbalanced diet. Fish that are only fed processed flake foods often suffer from malnutrition because these foods lack many of the essential nutrients aquarium fish need to thrive.
Mechanical filtration – The most basic type of filtration involving physically filtering solid wastes out of aquarium water. Examples of mechanical filters include wet/dry filters, canister filters and power filters.
Metabolism – The process through which an organism’s body derives energy from food through a variety of chemical reactions.
Metal halide lights – A type of light bulb that produces high-intensity light, white-spectrum light. This type of lighting is recommended for reef tanks as well as freshwater and planted tanks more than 24 inches deep. These lights require a ballast and may also require the use of an auxiliary fan or chiller because they produce a great deal of heat.
Methylene blue – A chemical compound commonly used to treat fungal infections but it can also be effective against freshwater Ich. Breeders often use this compound to treat the water in their spawning tanks to protect eggs against fungus.
Microalgae – Also called microphytes. Microscopic algae found in both freshwater and marine ecosystems. Examples of microalgae include hair algae and green algae.
Microscopic – A term used to describe objects and organisms that are too small to be seen with the naked eye. These objects and organisms are so small that they require a microscope to be seen clearly.
Minerals – A solid chemical substance that is naturally occurring and an essential part of a healthy diet for aquarium fish. Some of the most important minerals for aquarium fish are calcium and phosphorus.
Mouthbrooder – A type of fish which gathers the fertilized eggs into its mouth and guards them until they hatch. Even after hatching, the fry of mouthbrooding species may remain near or inside the mouth of the parent fish for some time.
Mulm – The organic matter that builds up in aquarium substrate. This substance is typically dark brown in color and is composed of organic waste, decaying plant matter and uneaten fish food.
Mutualism – A symbiotic (mutually beneficial) relationship between two organisms. An example of this is the relationship between Clownfish and Anemones.
Nauplius – The singular form of nauplii, the larval stage of crustaceans such as brine shrimp. Brine shrimp nauplii are often used as food for newly hatched fry.
Neon Tetra Disease − A disease affecting a variety of freshwater aquarium fish - not just Neon Tetras as the name would suggest. Neon Tetra Disease is caused by a parasite called Pleistophora hyphessobryconis. This parasite is a single-celled organism called a sporozoan. Treatment for this disease is usually a futile effort because in most cases the disease is too far advanced before symptoms are noticed.
Nitrate – (NO3) Non-toxic but still dangerous for fish at high levels. This substance is the final product of the nitrogen cycle, produced as a result of the oxidization of nitrite.
Nitrification – A term used to describe the oxidization of ammonia into nitrite which is then oxidized into nitrate. Nitrification is a fundamental step in the nitrogen cycle.
Nitrifying bacteria – Chemoautotrophic bacteria that consume inorganic nitrogen compounds. These bacteria play a fundamental role in the nitrogen cycle. Examples of nitrifying bacteria include nitrosomonas, nitrosococcus, nitrobacter and nitrococcus.
Nitrite – (NO2) Highly toxic for aquarium fish. This substance is produced during the nitrogen cycle as a result of the oxidization of ammonia.
Nitrobacter – A type of chemoautotrophic bacteria that plays an important role in the nitrogen cycle, converting nitrite into nitrate.
Nitrogen cycle – The process through which beneficial bacteria in the tank break down organic wastes into ammonia (NH3). The ammonia, which is toxic to fish, is then oxidized into nitrite (NO2) by beneficial bacteria called nitrosomonas. Next, nitrobacter oxidize the nitrite into nitrate (NO3), a substance that is not harmful to fish in small doses. In some cases, the process may be taken one step further by anaerobic bacteria which convert the nitrate into nitrogen gas which is then released into the air.
Nitrosomonas – A type of bacteria that plays an important role in the nitrogen cycle, converting ammonia into nitrite.
Nocturnal - Referring to organisms which are most active at night.
Novice – A term used to describe beginners in the aquarium hobby.
Nutrient – A chemical required by organisms to live and grow. Nutrients are taken from food and are used by the organism’s metabolism to extract energy for healthy growth of bones, tissues and organs.
Nutritional deficiency – See Malnutrition. A disease caused by a lack of essential nutrients in the diet of aquarium fish.
Omnivore – An organism which derives nutrients from both meat- and plant-based foods.
Organic debris – Organic matter such as fish feces and plant matter that has broken down in the aquarium. A portion of this debris is removed by the mechanical filtration component of the aquarium filter, but a large portion of it accumulates in the substrate of the tank as mulm.
Osmosis – The process through which solvent molecules pass through a permeable membrane, moving from areas of low concentration to high concentration. This process is particularly important in relation to salinity – aquarium fish that are introduced to an environment with higher salinity than they are used to will die quickly as a result of osmosis.
Osmotic stress – Also called osmotic shock. This term describes the adverse reaction caused by a sudden change in the solute concentration around the cell. In aquarium fish, this may occur as a result of a change in salinity.
ORP – Oxygen Reduction Potential. A measurement of the oxidizing power of tank water. This term is most often applied to reef tanks.
Ovovivipary – A method of reproduction in which the embryo develops inside the egg and is retained inside the mother’s body until hatching. This method of reproduction utilizes internal fertilization and the young are born live, but there is no placenta connecting the young to the mother. Ovoviviparous fish are often referred to as "livebearers". Guppies, Mollies, Platies, and Swordtails are examples of fish that are ovoviviparous.
Ozone – (O3) A triatomic molecule consisting of three oxygen atoms, often used in conjunction with protein skimmers. It can also be used to facilitate organic breakdown, reducing nitrite levels by converting it into nitrate.
Ozonizer – A device used to produce ozone (O3).
Parasite – An organism which feeds on another organism. Parasites typically attach themselves to, or invade, a host body, leeching nutrients at the expense of its host. Parasites are a leading cause of disease in both freshwater and marine fishes.
Peat moss – A type of moss belonging to the sphagnum genus which can be used to soften aquarium water and lower pH. Peat moss can also be used as a filter medium in brackish tanks.
Pectoral fins – A pair of fins located on either side of the body behind the head and gills. Pectoral fins help to control balance and direction of movement.
Pelvic fins – Also called ventral fins. A pair of fins located on the underside of the fish below the gills..
pH – A measure of how acidic or alkaline the water in an aquarium is. PH values are measured on a scale of 0 to 14 with a measurement of 7.0 being neutral. Measurements below 7.0 indicate acidic water while measurements greater than 7.0 are alkaline.
Pharyngeal teeth – Teeth located in the pharyngeal arch of the throat. These teeth are present in Goldfish, Loaches and some Cyprinids.
Pharyngeal jaws – A second set of jaws located in the throat containing a second set of teeth. The most notable example of fish possessing pharyngeal jaws is the Moray Eel, though many Cichlids possess them as well.
Phosphate – An inorganic chemical that can be toxic to fish in high concentrations. Excess phosphate in the aquarium can lead to uncontrolled algae growth.
Photoperiod – The amount of time aquarium lights are left on during the day.
Photosynthesis – The process through which carbon dioxide is converted into organic compounds using light as an energy source. Plants and certain types of bacteria utilize photosynthesis in the aquarium to convert carbon dioxide into oxygen.
Phytoplankton – Derived from the Greek word phyton, meaning “plant”. Microscopic algae living near the water surface where they have access to adequate light to facilitate photosynthesis.
Piscivore – A carnivorous organism that feeds primarily on fish.
Planktivore – An organism that feeds primarily on plankton. The blue whale is an example of a planktivore.
Plankton – Drifting marine organisms that inhabit the pelagic zone – the regions not near the bottom or shore. Plankton can be divided into three groups: phytoplankton, zooplankton and bacterioplankton. Phytoplankton are algae living near the water surface while zooplankton are animals that feed on other plankton.
Polyp – An individual, living unit of coral or anemone.
Powerhead – A submersible pump often used inside the aquarium to supplement the water movement produced by traditional aquarium filters.
ppm – Parts Per Million. A unit of measurement often applied to specific gravity and chemical levels.
Predaceous – A term used to describe organisms which prey on other organisms.
Protein – An essential element of a healthy diet for aquarium fish. Protein is required to maintain healthy growth of bones, scales and organs.
Protein skimmer – Also called a foam fractionator. A device most often utilized by marine hobbyists to remove organic compounds from tank water. These devices produce tiny bubbles which capture organic wastes like proteins and fatty acids, thus removing them from tank water.
Protozoa – A type of unicellular eukaryotic organism divided into four categories based on its means of motion: flagellates, amoeboids, sporozoans and ciliates.
Quarantine tank – A tank separate from the display tank used to isolate sick, injured or newly acquired aquarium fish. Quarantine tanks are generally set up to mimic the conditions in the main tank but are often bare-bottomed and sparsely decorated to facilitate easy cleaning.
Reactor – An isolated container often utilized in conjunction with sump systems to perform a certain role such as increasing oxygen content in the water.
Redox – Reduction-oxidation potential. The ability of tank water to facilitate biological reactions. Redox is often used as an indication of water quality and can be measured using electronic probes.
Refractometer – A device used to measure specific gravity of tank water, particularly in saltwater and reef aquariums.
Refugium – A tank used as an appendage to freshwater or marine tanks, sharing the same water supply. These tanks may be used to cycle water, to culture live rock, to stabilize water conditions or to isolate fish or other organisms.
Reverse-flow filtration – A type of biological filter which returns water to the aquarium near the bottom rather than at the surface.
Reverse osmosis – A purification process for aquarium water by which large molecules and ions are removed by applying pressure to the water on one side of a permeable membrane. As the water passes through the membrane, the impurities are left behind.
RO unit – Reverse osmosis unit. A device used to purify water by forcing it through a semipermeable membrane which traps minerals and impurities.
Rotifer – a type of microscopic organism often used as food for aquarium fish.
Salinity – The measure of the amount of salt in water, measured in parts part thousand (ppt). Seawater has a natural salinity around 35 ppt. Salinity can be measured using a refractometer.
Scavenger – An organism which feeds on dead and decaying organic matter in the aquarium. Snails are a common example of scavengers in the home aquarium.
Self-ballasted – A term used when describing certain types of aquarium lights. Light bulbs that are self-ballasted have the ballast, or power source, built into the bulb whereas externally ballasted bulbs require a separate power source.
Semi-aggressive – A term used to describe fish which can be kept with other species of fish, but which may display violent tendencies in certain situations. Males of semi-aggressive species, for example, may be territorial and could become antagonistic toward other males of the same species.
Septicemia – A serious condition in aquarium fishes involving severe inflammation as a result of infection.
SHO lights – Super High Output lights. A type of lighting often used in greenhouses and hydroponics gardens that can also be utilized in planted tanks. These lights put out high-intensity light brighter than standard fluorescent bulbs.
Silicone sealant – A transparent adhesive used to repair leaks in glass aquariums. This substance can also be used to construct 3-D tank backgrounds and to attach rock and corals.
Siphon – See gravel vacuum.
Slime coat – The protective layer of mucus that forms on the scales of fish. This coating reduces friction during swimming and helps to defend fish against disease.
Soft water – Aquarium water having a low mineral content. Measured on the scale of general hardness, soft water typically measures between 0 and 60 mg/L and between 0 and 3.36 °dH.
Spawning – The reproductive process utilized by most egg-laying species of fish. During this process, the eggs and sperm are released, either simultaneously or separately, by the female and male fishes. The word “spawn” can be used as a verb or as a noun referring to the eggs and sperm released into the water during the spawning process.
Spawning site – The location within the aquarium a fish or pair of fishes has chosen for depositing their eggs. Some species deposit their eggs on exposed spawning sites like rocks, plants and aquarium glass while others bury their eggs in concealed spawning sites within caves or under aquarium substrate.
Spawning tank – Also called a breeding tank. A separate tank used for breeding purposes designed to isolate a breeding pair of fishes. The water conditions in a spawning tank can be controlled and manipulated as required by the species of fish being bred.
Species – The most basic unit of biological classification. A species is a group of genetically distinct organisms which produce fertile offspring after breeding.
Species tank – An aquarium housing only one species of fish. Many aggressive species of fish are best kept in species tank to avoid conflicts with other tank inhabitants.
Specific gravity – The ratio of the density of tank water to the density of pure water – used as an alternative measurement of salinity. Pure water has a specific gravity of 1.000 and seawater has a natural specific gravity around 1.025.
Splash guard – A device which prevents water from the aquarium from splashing or condensing on aquarium light bulbs. Splash guards come in a variety of forms, but the simplest and most common form is a glass or plastic lid placed on top of the aquarium to keep water from escaping.
Sponge filter – A type of aquarium filter which utilizes both mechanical and biological filtration. Sponge filters are very simple, consisting mainly of a sponge and a powerhead connected to an intake tube.
Spore – The means through which many bacteria, algae and fungi propagate. A spore can develop into an entirely new organism through the process of cellular division.
Sporozoan – One of four types of parasite. Sporozoans use a special organelle called an apicoplast to penetrate host cells and parasitize them.
Stock – A term sometimes used to refer to the inhabitants of an aquarium. This term can also be used as a verb meaning to “add” fish to a tank or “fill” the tank with inhabitants.
Strain – A variant or sub-type within a particular species of aquarium fish. For example, many species of fish come in a variety of colored strains - like the Zebra Danio which can also be found in an ornamental “golden” strain.
Substrate – The substance placed on the bottom of an aquarium. Substrate can be both decorative and functional, serving to increase the aesthetic appeal of a tank while also providing a place for beneficial bacteria to grow. Popular substrates for freshwater tanks include sand and gravel.
Suction – The upward flow of water induced by an impeller in most aquarium filters, pulling the water up through the intake valve.
Sump – A type of aquarium filter that is typically located below the tank. A sump increases the water capacity of the tank and can also be used to hold other equipment like heaters and protein skimmers.
Swim bladder – A gas-filled organ in fish that controls buoyancy and acts as a stabilizing agent. Swim bladder disease is a condition characterized by the build-up of bodily fluids which conflicts with the swim bladder, resulting in difficulty swimming.
Systemic infection – An infection or disease that affects the internal organs and tissues of aquarium fish. The symptoms of these infections are often difficult to identify and may not become evident until the disease has progressed into the final stages.
Tank capacity – The volume of water any given tank is capable of holding, often measured in gallons.
Tank divider – A physical barrier that can be inserted into the aquarium to divide the tank into multiple sections. Tank dividers are often made from mesh-like materials so filtered water can be distributed throughout all sections of the tank. These devices are often used to isolate aggressive or injured fish and can also be used to separate parent fish from their fry.
Tankmates – Referring to the other fish being kept in the same tank with the fish in question.
Tannins – Biomolecules released by plants, particularly when submerged. Water containing a large quantity of submerged vegetation is often stained to a tea-color with tannins and may have a relatively low pH value.
Taxonomy – The science of identifying, naming and classifying different species. The taxonomical classification of an organism progresses from the domain, the highest taxonomical rank, down to the specific species.
Trace elements – Elements that occur in very small quantities, less than 100 ppm, in tank water. Some of the trace elements commonly found in aquariums include iodine, calcium, strontium, lithium and barium.
Trickle filter – A type of aquarium filter in which tank water trickles down through some type of filter media. Filter media for this type of filtration system typically includes plastic balls, rocks, lava, peat moss and polyurethane foam.
Tubifex worms – A type of segmented worm naturally found in the sediment of rivers and lakes. These worms are a popular food for aquarium fish in frozen and freeze-dried form. Many aquarists avoid these worms, however, because if they are harvested from polluted waters they can potentially contaminate the tank as well as aquarium fish.
Turnover – The rate at which tank water flows through an aquarium filter. For freshwater tanks, a turnover of the entire tank volume approximately four to five times per hour is recommended. Higher turnover rates are often necessary in reef tanks.
Ulcer – A break in a bodily membrane, a portion of tissue that has decayed or died.
Ultraviolet light – A type of short wavelength light.
UV Sterilizer – Ultraviolet sterilizer. A device used to kill bacteria, parasites and algae in tank water. These devices are often used in conjunction with aquarium filters – the water being released from the filter is passed over the UV lamp.
Undergravel filter – A type of aquarium filter that is installed beneath the substrate – water is sucked down through the filter plate and released via outlet tubes. This type of filter utilizes both mechanical and biological filtration.
Venturi – A type of valve used in some protein skimmers which produce air bubbles by drawing air into a stream of flowing water.
VHO lights – Very high output lights. A type of fluorescent aquarium lighting that produces high-intensity light up to three times as strong as standard fluorescent bulbs. These lights require special fixtures and ballast systems and may produce enough heat to require an auxiliary fan or chiller. VHO lights are recommended for planted freshwater tanks and for reef tanks less than 24 inches deep.
Vitamins – Organic compounds that are required by aquarium fish as essential nutrients for healthy growth and function. Some of the most important vitamins for aquarium fish include Vitamins A, C, E, B1, B2, B5 and B6.
Water change – The process of removing a portion of the tank water volume and replacing it with fresh water.
Water chemistry – Also referred to as water parameters. The physical and chemical characteristics of tank water including pH, alkalinity, temperature, salinity and chemical levels. Maintaining stable water chemistry is essential for the health of aquarium fish.
Watt – A unit of power used to measure the rate of energy conversion in light bulbs. Basically, it's a measurement of the amount of energy being used by a light bulb.
Wet/dry filter – A type of aquarium filter that utilizes biological filtration, exposing the water to air to aid in the nitrification process.
Whiteworms – A type of worm often used as food for aquarium fish in both live and frozen form. These worms are high in lipids and are favored among breeders for use in conditioning fish for spawning.
Yolk sac – A sac attached to newly hatched fry that provides nourishment during the first few days of life. Once this sac has been absorbed the fry will be free swimming and ready to consume real food.
Zeolite − A white granular chemical filtration media typically used in canister filters to remove ammonia.
Zooplankton – Microscopic animals floating in seawater. Included in this category are waterborne crustaceans, corals, jellyfish, anemones and fish larvae.
Zooxanthellae – Microscopic organisms living symbiotically with some species of coral, clam and sea sponge. These organisms are responsible for the bright pigmentation of many corals.