Fluval Bio Foam Pads are uniquely designed to fit Fluval FX canister filters. The Bio-Foam combines mechanical & biological filtration. Ist pore structure increases surface area for the growth of helpful nitrifying bacteria. In addition, it traps small particles that are not retained by the first stage of foam filtration.Fluval Bio Foam Pads FX Series+ enhances biological filtration+ effectively captures small particles and debris+ for freshwater and marine water enviroments+ rinse every 3 months, replace every 6 months+ for Fluval External Filters FX4/5/6+ Package content: 2 pieces+ Fluval No. A239