These Fluval poly foam pad filters are used as the Stage 2 mechanical filter media in the Fluval C4 Aquarium Power Filter System to decrease the occurrence of fine particles. This assortment pack also includes the Stage 1 Fluval C4 Foam Pad for the elimination of larger particles. These Fluval foam pad replacements for the Fluval C4 Aquarium Filter are made with porous foam to capture large particles, debris, and impurities such as dirt, sediment, sand, rust, hair, and more. Fluval foam filters lessen the amount of particulate matter in your aquarium tp give your aquatic pets a better living environment. They are especially designed for heavily populated aquariums. Pad combines two stages of mechanical filtration. Stage 1 captures large particles within the porous foam. Stage 2 traps fine debris within the dense polyester layer. Sizes fit Fluval C2, C3, or C4 power filters. Instructions for Use: Fluval C4 foam pad filter replacements should be changed approximately every 2-3 months, though they can be rinsed clean between filter changes to extend filter life.