An under gravel filter or UGF is a biological aquarium filter. It consists of plates that sit under the gravel, which are fixed with feeder tubes that rise up to just below the surface. Airstones or powerheads are inserted into the feeder tubes to draw water from underneath the plates. This pulls water down through the gravel bed, ergo the entire substrate acts as a medium for hosting the positive bacterial colonies that are responsible for the biological filter. The result is that the full volume of tank water is continually being circulated through the gravel or substrate to be purified by the biological filterbed.
Another benefit of the UGF is surface agitation. Proteins can gather on the surface forming a thin membrane that effectively blocks the exchange of air molecules with water. When this happens, oxygen becomes depleted and fish can suffocate. Agitating the surface of the water keeps a membrane from forming and allows continuous exchange of air molecules. The water drawn up through the under gravel filter feeder tubes is expelled near the surface, providing this service.
UGF feeder tubes can be fitted with simple airstones that provide a slow gentle current, or powerheads that provide a strong current. Each has advantages depending on need. Not all fish like a strong current. In fact it can be exhausting and detrimental to certain types of fish. Other fish require current to be healthy.
As a rule, fish that are streamline in shape or torpedo-shaped like neon tetras normally enjoy a little current, relatively speaking and do well with powerheads; while those that are "flat" or disc-shaped, like gouramis exhaust easily in current. This is because the flat body of a disc-shaped fish catches the current much more than a torpedo-shaped fish. For tanks that host disc-shaped fish, airstones are probably a better choice.
Breeding tanks where small fry are kept are another example of when airstones would be more appropriate. Airstones might also be a good choice for a planted tank that has finer gravel or soil. Powerheads might pull the water too quickly through the substrate making it difficult for the roots of plants to absorb nutrients, and fine soil might also clog the under gravel filter making in ineffective.
If you have a fish-only tank, however, or few plants, and your fish like current, powerheads are a good choice. A powerhead is a motor with a magnetically powered spinning impeller. The motor is encased in watertight housing, and the bottom of the powerhead is cone-shaped to insert into the under gravel filter feeder tubes. The powerhead sits on top of the feeder tube just under the surface of the water, and pumps water up through the tube at a high rate, expelling the water in a stream. With a powerhead at each end of a tank feeding down to the under gravel filter, significant currents can be created which provides the stimulation necessary for many fish.
Another benefit of using powerheads is that the draw is powerful enough to pull debris into the gravel until it can be vacuumed. This keeps the tank looking clean between vacuuming. Powerheads are also virtually silent, whereas some air pumps are quite noisy.
Proper maintenance of the UGF is necessary to maintain a healthy aquarium. If heavy debris is allowed to collect under the plates and the power should go out, the debris could cause a deadly ammonia spike. To avoid potential problems, when vacuuming simply push the vacuum tube down through the gravel until it hits the plates. Hold it flat against the plates until the water rising up through the tube becomes clear. If gravel is drawn up the tube, regulate the suction by squeezing the tubing or partially placing your finger over the end that is expelling into the drain bucket. Move the tube to a new spot and continue this process across the entire bottom of the tank, or do half the bottom at each cleaning. This will ensure debris does not build up.
In addition to the UGF, a tank will likely have mechanical and chemical filtration. These are provided by canister filters, hang on tank filters, and carbon or specialty resins.
Under gravel filters play a valuable role in the overall filtration of almost any aquarium. With proper maintenance an under gravel filter will keep your tank looking great, the water oxygenated, and the gravel bed full of positive bacterial colonies.