Wet Dry filters are recommended for all types of freshwater tanks and for saltwater fish only tanks. Wet Dry filters work by mixing air and water mix as the water from the overflow box 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. The Bio Bale media provides 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.