The Filter Pad Set for the Eheim Pro 4+ Canister Filter comes with fine and course pads for mechanical and biological filtration. These can be used in saltwater or freshwater aquariums, and has exceptional colonization conditions for bacteria cultures. Because these pads are constructed with porous material, water is able to distribute throughout the pad evenly allowing the pad to trap diverse dirt particles. After a short start-up period, purification bacteria settle in the specially structured porous foam material. This provides intensive natural decomposition of harmful and foreign substances, making your aquarium a much safer environment for your aqua life. These pads can be cleaned and reused numerous times. When you are washing these filter pads, be sure to rinse and squeeze them out, making sure the bacteria cultures are not completely destroyed by over-washing.