The Fluval 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 for External Filter Series 04/05/06/07+ enhances biological filtration+ effectively captures small particles and debris+ for freshwater and marine water enviroments+ rinse every 3 months, replace every 6 monthsfor Fluval External Filter: 104, 105, 106, 204, 205, 206, 207 -Fluval No.: A236for Fluval External Filter: 304, 305, 306, 404, 405, 406, 407 -Fluval No:. A237, 2 pieces pack