Fluval Foam inserts perform mechanical filtration, the crucial first stage in the filtration process. Foam pore size provides efficient water flow with less clogging for long lasting filtration of the aquarium water.Fluval Foam Pre Filter Inserts Series 04/05/06/07+ captures all dirt particles & debris+ custom fit to prevent bypass+ enhances biological filtration+ clogs less, lasts longer+ for freshwater and marine water enviroments+ can be rinsed several times+ recommended replace interval: every 6 months+ Package content: 2 piecesfor Fluval External Filter: 104, 105, 106, 107 -Fluval No.: A220for Fluval External Filter: 204, 304, 205, 305, 206, 306, 207, 307 -Fluval No.: A222for Fluval External Filter: 404, 405, 406, 407 -Fluval No. A226