There are 2 types of air-filtration used. Both use an ‘Air Pump’ to provide ‘air’ to ensure filtration.
Under-Gravel filtration relies upon a plastic base where particles are drawn down into the substrate via an ‘uplift’ system. The beneficial bacteria live within the substrate which convert the harmful substance (Ammonia) given out by the fish into the more harmless Nitrate.
Air powered ‘sponge’ filtration uses a simple technique of utilising the rising air bubbles to draw in the water contaminated by ‘fish waste’ (ammonia and uneaten food) into the sponges that harbour beneficial bacteria that use the ‘Nitrogen’ cycle to convert the harmful substances into far less harmful Nitrate (NO3).