Undergravel filters are an alternative method of aquarium filtration that sits beneath the substrate (or gravel) in your tank, eliminating the need for bulky filters dangling on the back wall. There has been some debate surrounding undergravel filters due to some factors. While using one may reduce maintenance needs by eliminating frequent filter cleaning sessions, once they are in your tank and your substrate and decor have been placed around them they cannot be easily reached and require proper upkeep; otherwise they could pose significant problems to aquarists.
How Undergravel Filters Work
To install an undergravel filter, start with an empty tank. An undergravel filter consists of a plastic plate which will rest at the bottom of your tank. Attached to the plate are uplift tubes designed to move water by either using an air pump or powerhead water pump, pulling water down through gravel layers under the plate before pulling it back up the tube again. Water flowing through the gravel creates a water current which pulls waste down while simultaneously encouraging aerobic bacteria growth, turning the entire gravel bed into an effective biological filter. Waste and debris will pass through the substrate and under the filter plate; any that remains must be regularly vacuumed out using deep gravel vacuuming with a siphon.
Undergravel filter plates should cover the entire bottom of a tank, so multiple filter plates will likely be necessary in larger tanks to provide suction from different locations and pull water through gravel layers and under the plate. Suction is created with an air pump attached to an air stone situated inside of an uplift tube. For optimal results, bubbles from this air stone should have fine, strong streams in order to efficiently draw in water via rising bubbles. That creates suction with water moving down through the gravel, under the filter plate and up into the tube with bubbles. There are also water pumps called “power heads” designed to sit atop an uplift tube and pull water through filter plates more effectively and efficiently than using air stones.
Some undergravel filters feature cartridge components–typically containers filled with activated carbon or foam sponges–that attach to the uplift tube spout for use. You do not have to incorporate this feature in your undergravel filter except in certain setups listed below.