Aqueon Replacement Filter Cartridges Fits Aqueon QuietFlow Filter: LED PRO Size 10 and E Internal 20 gallon Ready-to-use cartridge contains high quality activated carbon that keeps water clean, rinse before replacing Installs in seconds: align cartridge bottom key slot for an accurate fit inside the Filter For optimal performance, change approximately every 4 to 6 weeks Do not clean Bioholster when replacing the cartridge as it contains beneficial biological bacteria