Hydor Professional External Canister Filter Media, Zeolite For Use With Any Size Hydro Professional Canister Filter; Can Also Be Used With Other Canister-Style Filters Box Contains 550Gm Pouch Chemical Filtration for Freshwater Aquariums, specifically for removing ammonia, dyes and drug residues from aquarium water Zeolite Should Be Used As A Middle Stage Of Filtration It Is Recommended That Zeolite Should Be Changed Out Every 1 To 3 Months