Fluval U-Series Foam Filter Pads. The Fluval foam pad replacements are made with porous foam to trap large particles, debris, and impurities such as dirt, sediment, dust, sand, rust, hair, limescale, and more. They reduce the amount of particulate matter in your aquarium to help your aquatic pets thrive better and are especially suited for heavily populated aquariums. Should be changed approximately every 2-3 months, though it can be rinsed clean between filter changes to extend filter life.