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. After passing through the filter plate, the water is drawn up through a tube, by means of an air pump or electrical water pump, to the water's surface, where it is pumped back into the aquarium. An electrical water pump is most often used in larger tanks needing greater water flow.
Plants use nitrate, and it is mostly harmless to fish.
The drawback of the under-gravel filter is that the dirt gets trapped in the gravel. This second process also requires special equipment, also available at a pet store. Sponge Filter Sponge filters provide another relatively inexpensive method of biological filtration. A special pulls water through a porous foam impregnated with bacteria. The bacteria colony neutralizes the ammonia.