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 Internal Filters U Series+ captures all dirt particles & debris+ custom cut for optimal fit+ enhances biological filtration+ clogs less, lasts longer+ for freshwater and marine water enviroments+ can be rinsed several times+ recommended replace interval: every 6 monthsfor Fluval Internal Filter U1, Fluval No.: A485, content: 1 piecefor Fluval Internal Filter U2, Fluval No.: A486, content: 2 piecesfor Fluval Internal Filter U3, Fluval No.: A487, content: 2 piecesfor Fluval Internal Filter U4, Fluval No.: A488, content: 2 pieces