Cascade filter replacement cartridges from Penn-Plax work to keep your aquarium water clean & clear. Each pack contains 3 disposable filters specially designed to fit your Cascade 300 hang-on power filter. Each cartridge uses a poly-fiber floss barrier to screen out particulate matter & debris while the activated carbon works to remove ammonia & nitrites keeping your aquarium clean & healthy. Each easy-to-replace disposable cartridge is perfectly sized to fit your Cascade 300 hang-on power filter. For use on Cascade 300 hang-on power filters only.