JUWEL Bio pad is used to catch coarse particles of dirt in the water. As a mechanical pre-filter, it protects the filter from rapid soiling and means that you don't have to clean the other filter media as often. JUWEL Bio Pad filter floss consists of fine, residue- free polyester material and is suitable for use in both saltwater and freshwater aquariums. It comes in four sizes, in packages of up to 5.