It then sends the water back into the aquarium. It does not remove dissolved ammonia or microscopic bacteria and algae. Aeration is a better method of tank cleaning in this case, as some medicines deplete a tank's oxygen level. The under-gravel filter uses the gravel at the bottom of the aquarium for cleansing. Aside from the pump, most of the equipment is buried beneath the gravel.
Water passes through the gravel bed, where a bacteria colony resides.
This bacteria is maintained by the constant oxygenated water flowing through the aquarium. An under-gravel filter can never be switched off, because when the water flow stops, this bacteria layer starts to die. The water then goes through a biological filter plate, which covers the whole base of the tank. If used in a tank with digging or burrowing fish, the under-gravel filter plate needs to be protected. Place a sheet of netting between gravel layers slightly above the filter to prevent the fish from damaging it.