Under gravel filters (UGFs) consist of a grid of plates concealed between the gravel or substrate and the base of the aquarium. Water is drawn through the substrate, under the filter plates and returned via one or more uplifts. The filter can be powered by an airpump, or a powerhead - an electric pump which can be fitted to the uplift pipe. Aquariums which require a gentler current should opt for an air driven system.

The substrate is the filter media and for maximum effectiveness should be 3-4 inches deep. Never use sand as a medium in a UGF as it clogs and compacts quickly. The substrate acts mechanically, biologically and depending on the substrate material, chemically. The great advantages of a UGF are, the filter is the entire aquarium floor and thus debris cannot escape processing. Additionally the vast surface area inherent with substrate can sustain massive bacterial colonization. The main disadvantage of an UGF is that when the system eventually becomes clogged, cleaning involves major upheaval. Interim cleaning with a gravel siphon minimises this problem but many aquarists solve it by another novel method - reversing the flow.

Reverse flow UGFs utilise an external power filter as a mechanical pre filter and attach the outlet to the uplift pipe so that water is flowed down the uplift and up through the substrate. This unfortunately means that the main source of biological activity is now in the pre-filter, which negates the main advantage of a conventional UGF. Reverse flow systems also create negligible surface turbulence which may result in oxygen depletion and the need for additional aeration.

In my experience, UGFs are suited to aquariums smaller than 4 feet (50 UK gallons) and uplift pipes are best positioned every 18 inches along the back of the tank. It is also worth mentioning that it is allegedly impossible to grow plants if an UGF is employed. The theory is that plants can not tolerate a flow of water past their roots. While this may be true for certain plants most hardy species will still thrive.