The Best Under-Gravel Fish Filters

In essence, under-gravel filters work by turning the gravel bed into biological and mechanical filter media. Under-gravel filters have three key components. One or more plastic, perforated plates sit below the gravel in the tank and creates a layer of water underneath the gravel. A tube sits vertically in the tank, and connects to either an air stone or a water pump. The stone or pump causes water to flow up through the tube. In doing so, the tube draws water from the space below the gravel, which in turn pulls water through the gravel bed. This movement pulls particulate matter deeper into the gravel, and provides a high amount of surface area to host beneficial bacteria.

Historically, under-gravel filters were the only type of filters that were effective and readily available. Many hobbyists who began keeping fish at this time, still rely on under-gravel filters to keep their aquariums clean, even though other options are available. Conversely, new hobbyists often select a canister that hangs on the back of the tank or wet-dry filter to maintain their tank filtration. While under-gravel filters require less maintenance than most other filters, hobbyists must clean the gravel periodically to ensure adequate water flow, and conduct weekly, partial water changes.

There is no shortage of under-gravel filter manufacturers, and many budget models exist. While you may save a few dollars by selecting such a model, you will get better value from a premium under-gravel filter. Some hobbyists prefer air-driven units, while others prefer systems powered by water pumps. Each type has pros and cons: Air-driven systems often work with an optional filter cartridge, while water-driven systems usually produce higher flow rates. Understand that under-gravel filters are not appropriate for all tanks. You should opt for a different type of filtration if you have a reef tank, a heavily planted tank or one with slopping gravel.

Three under-gravel filters are effective and popular among hobbyists; not surprisingly, they are all similar. Lee’s Premium Undergravel Filter is an air-powered unit that can accommodate optional chemical filtration cartridges. Additionally, the filter’s plates are made of crack-resistant plastic, and the design includes excluders to prevent fish from accessing the apparatus. The Penn Plax Undertow and Penn Plax ClearFree Premium under-gravel filters are similar in basic design and features. The primary differences between the three products involve subtle details of the filter plates.