Penn Plax Cascade 400 Internal Filter Replacement Cartridges Keep your water clean and clear with the replacement bio sponge for use in the Cascade 400 GPH Model CIF2 Internal Filter. The bio sponge filter removes ammonia and nitrites from your aquarium tank keeping your aquarium clean; healthy; and odor free. Filter sponge optimizes the colonization of beneficial aerobic bacteria which prevents the water from becoming cloudy and harming fish. Perfectly cut to fit into the Cascade 400 Internal Filter tray without any gaps which could allow pollutants to re-enter the water.