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 FX Series+ captures large particles+ custom fit to prevent bypass+ enhances biological filtration+ for freshwater and marine water enviroments+ can be rinsed several times+ recommended replace interval: every 6 months+ for Fluval External Filters FX4/5/6+ Package content: 3 pieces+ Fluval No. A228+ for Fluval External Filters FX2+ Package content: 2 pieces+ Fluval No. A227