Sponge filters are the most basic of filters. Air, provided by an airpump, draws water through a sponge and gently returns it via an uplift pipe to the aquarium. These filters are available in a range of sizes to suit lightly loaded tanks of up to 2 feet (20 UK gallons) but are most commonly used in breeding tanks. The draw of water into the sponge is so minimal that even fry can escape. Additionally, the exposed filter media allows fry to feed off the micro-bacteria that will develop on the sponge. Sponge filters act mechanically and biologically.