These high-quality sponge filters are excellent for breeding tanks, fry tanks or fish aquariums. They utilize a hydrophilic sponge that creates an environment for cultivating a high density of nitrifying bacteria. Designed to operate with air, they provide excellent mechanical and biological filtration. Four Models Available: - Oxygen 1, up to 25 gal. - Oxygen 2, up to 50 gal. - Oxygen 4, up to 75 gal. - Oxygen 5, up to 30 gal.