The undergravel filter has its drawbacks. But some aquarists swear by it. Is the undergravel filter a good investment, or a waste of time? You decide.


An undergravel filter is merely one of the many varieties of aquarium filters on the market. These filters are relatively simple in design, consisting of little more than a plastic grate that is placed under the gravel and attached to an intake tube. Because they are so simple in design and do not require the use of filter media, undergravel filters are relatively inexpensive, making them a popular choice among novice aquarists. Before you go out and buy an undergravel filter, however, there are a few things you should know.

How Undergravel Filters Work

undergravel-filterAn undergravel filter generally consists of a plastic filter plate which is placed on the bottom of the tank before the gravel or other substrate is added. This slotted filter plate is attached to one or more intake tubes which pull aquarium water down through the filter plate. This process is generally driven by air displacement – air stones are inserted into the base of the intake tubes and, as the air bubbles force water out of the tubes, negative pressure is created under the filter plate. In response to the negative pressure, aquarium water filters down through the gravel and into the intake tubes. Alternatively, a powerhead can be attached to the intake tubes of an undergravel filter in order to provide more powerful suction and increased circulation.

Type of Filtration Offered By An Undergravel Filter

The main function of an undergravel filter is to provide biological filtration. Because beneficial bacteria feed on the organic waste and debris that accumulates in the substrate of your tank, an undergravel filter creates a perfect environment for these bacteria to grow. As aquarium water is filtered through the gravel it is also being circulated and oxygenated, making it an even more desirable environment for bacteria. Some undergravel filters can be adapted for chemical filtration through the use of an activated carbon cartridge which may help to remove toxins and dissolved wastes. Because the filter cartridges for undergravel filters are so small, however, they provided very limited chemical filtration.

Risks Associated with Undergravel Filters

goldfish xs 1337252Over time, solid waste and debris will build up at the bottom of your tank because the aquarium water is being filtered down through the gravel. As waste accumulates, the ammonia level in your tank will rise. Ammonia is a substance toxic to fish so, unless you take quick action to remedy high ammonia levels, your fish could die. In addition to resulting in increased waste accumulation along the bottom of the tank, undergravel filters can also cause the gravel to become packed. When this happens your undergravel filter may stop working entirely. If you do not monitor your aquarium closely, the failure of your filter could go unnoticed for quite some time and your fish could suffer in the meantime.

Undergravel filters have also been known to cause problems in planted tanks. Because an undergravel filter works by drawing aquarium water down through the gravel and into the filter plate, rooted aquarium plants could be affected. Not only can the roots of aquarium plants clog an undergravel filter but the suction of the filter could actually damage the plant roots and the excess water circulation could hinder their growth. If you want to keep live plants in an aquarium that has an undergravel filter it is wise to place a barrier between the plant roots and the filter plate. You may also choose to pot your plants as a means of avoiding conflict between the roots and the filter plate. Avoid this problem altogether by choosing an alternate means of filtration, such as a canister or hang on the back filter.

How to Make the Best Use of an Undergravel Filter

Undergravel filters are generally not recommended as the sole source of filtration in an aquarium. This type of filter can be beneficial, however, as a supplemental source of biological filtration. If you do plan to use an undergravel filter in your tank, keep in mind that because a large volume of aquarium water is being filtered through the gravel you may have a higher build-up of waste in your substrate than you might otherwise experience. This being the case, it is especially important to keep up with your routine water changes and to vacuum your gravel regularly to prevent excessive waste build-up which could lead to toxic levels of ammonia and nitrites in your tank. In addition to regularly cleaning your tank, avoid overfeeding your fish. If you only feed your fish as much as they can consume in 3 to 5 minutes there will be less food leftover to sink to the bottom of the tank - which means less waste to clog the undergravel filter.

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