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https://www.swelluk.com:443/fluval-flex-foam-filter-insert/ 
Fluval Flex Foam Filter Insert
Replacement black foam filter insert for the Fluval Flex tank, 57L, and the Fluval Flex 123l, meaning you can replace this as required to keep your tank looking at its very best.
Product Information
This replacement foam is for the 57l Fluval Flex tank, meaning you can replace the foam as required to ensure your tank is kept clean and in its best condition. These can be replaced as required, when the foam blocks up.
This mechanical media traps particles of dirt within the tank, keeping it looking clearer, and also allows for the growth of bacteria for optimum biological filtration as well.