Aquarium Air Filters are one of the most basic, simple and cost effective types of filtration.
The most common air filter is the aquarium sponge filter. They are very easy to set up - just use some airline to connect the sponge filter to the air pump and away you go. As the bubbles rise to the top of the tank, water is drawn through the sponge. The sponge is now acting as a great biological filter allowing the good bacteria to break down the fish waste as new water moves through it.
Aquarium sponge filters are also good mechanical filters. By trapping the waste and leftover food around the sponge, it allows baby fish, shrimp or small fry to feed off any uneaten food. Sponge filters can be used for small tanks, quarantine tanks, isolation tanks for sick or injured fish, shrimp tanks or even as an extra filter for a heavily stocked tank.