Carefree Fish Aquarium USB Sponge Filter Quiet for 20~60Gal Fish Tank for Saltwater and Freshwater with Biochemical Filtration Balls and Fluidized BedsBrand: Carefree Fish Color: Black Features: MultifunctionalIncrease the flow of water, increase oxygen, adsorption magazine, cultivation of nitrifying bacteria. The water in the fish tank will produce toxic ammonia and nitrite. Simple physical filtration cannot remove these toxic substances. To improve the water quality, a nitrifying bacteria system must be established for the fish tank. Changeable Filter Sponge: Sponges can be cleaned and used