Provides mechanical straining of particulate waste within the filter. Prevents waste from prematurely clogging other filter media in the module stack. Designed specifically for use in Hagen's Fluval Canister Filters, these Foam Blocks effectively strain particulate waste, so that it doesn't clog the other filter media in the module stack. Its primary purpose is mechanical filtration, but biological activity will also occur within the pore structure. Rinse foam monthly in aged aquarium water and replace when the pore structure begins to break down.