The Cascade 500 Canister Filter Replacement Bio Sponge helps keep your aquarium water clean. The sponge removes ammonia and nitrites from you aquarium water keeping your tank healthy. This bio sponge also optimizes the colonization of beneficial aerobic bacteria which prevents cloudy water and excess CO2 which can be harmful.Made specifically to fit the Cascade 500 canister filter; this replacement bio sponge fits securely without gaps that lead to weaknesses in filtration strength. An exact fit keeps debris and contaminants from getting through your filtration system and back into water. Made of a durable poly-fiber.