Phosguard rapidly removes phosphate and silicate from marine and freshwater aquariar it is not recommended for phosphate buffered freshwater phosguard is highly porous for high capacity and bead shaped for optimum water flowr it outperforms all competing products 500ml easily treats over 150 gallonsr directionsr for best results phosguard should be placed so as to maximize the flow of water through it it may be used in a canister filter box filter or any high flow area of a trickle fil