Features:----- Specifically designed for Fluval 306/307 & 406/407 filters, Effective at removing and reducing nitrite, Effectively traps floating particles for clear water (debris, excess food, and waste), Supports beneficial bacteria for a healthier aquatic environment, Custom fit to increase filtration efficiency, For best results, replace monthly or as required after testing ... https://www.maplepets.in/product/nitrite-remover-for-306-406-307-407-canister-filter-6-pack/