Finest-Filters compatible replacement foam and polycarbon filter cartridges are made from the same material as the Fluval originals.
These filter pads are made to fit the Fluval U4 range of internal filters
The plain filter sponges contain the biological cultures, biologically and mechanically filtering the aquarium water.
The polycarbon filter cartridges remove small particles from the water, and the activated carbon chemically filters the water of impurities.
Please Note: This is not a Fluval Product. This is a Finest-Filters alternative which serves the same purpose as the original brand.