Hydro-Sponge II Pro filters provide both mechanical and biological filtration. The Pro line is constructed of reticulated foam which doubles its efficiency. Aerobic bacteria colonize the sponge providing a bacterial substance to facilitate the nitrogen cycle. In addition, the Hydro-Sponge traps larger waste particles mechanically cleaning the water without trapping baby fish. Other sponge filters can't compare to the amount of water the Hydro Sponge can move with the one inch diameter lift tube. The sponge is held on a heavy base to keep the unit stable and the raised sponge design prevents dead space beneath. Ideal for use in hatcheries and breeding facilities. Hydro-Sponge filters can be operated with either and air pump and stone, or a suitable powerhead. The Hydro II is rated for aquariums up to 20 gallons.