Finest-Filters 8 x Compatible Carbon Foam Filter Pads to fit Fluval U4 Range of Internal Filters Our carbon filter pads are intended as an alternative to the poly/carbon cartridge, and offer excellent chemical carbon filtration in the aquarium. These filter pads are cut to fit the Fluval U4 range of internal filters. Please note: There is no poly side to these filters. They are just Activated Carbon impregnated foam. This is not a Fluval Product. This is a Finest-Filters alternative which serves the same purpose as the origional brand.