The value of a biological filter depends on how well it performs the tasks of oxygenation, ammonia conversion and gas exchange on a long term basis. In the wet/dry filter, air and water mix as water trickles over an exposed filter media such as Bio Bale .Biological filtration and gas exchange occur as the water flows in a thin layer over the media surface. (CR Series filters provide optimum growing conditions for bacteria which oxidize nitrite and toxic ammonia.) The rapid oxidation of ammonia and nitrite helps to maintain a stress free environment for aquatic life.