Penn Plax Cascade 300 Filter with Internal Replacement Cartridge, 6 Total Cartridges (3 Packs with 2 cartridges per pack) 6 Total Cartridges (3 Packs with 2 cartridges per pack) Convenient, no-fuss replacement cartridges for your Cascade 300 internal power filter Activated carbon removes ammonia & nitrites keeping your aquarium healthy Easy-to-replace disposable filters