QTY: 2PCS PFE Replacement Filter Inserts with Activated CarbonThe PFE cartridge layer is made of strong poly-fiber and collects organic debris for easy removal. Filled with AquaTop’s Activated Carbon, these replacement pads are extremely easy to use and install and help keep aquarium water crystal clear and free from odor.NOTE: AquaTop recommends replacing the Filter Insert filled with Activated Carbon every 4-6 weeks. Aquariums with heavier livestock loads may require more frequent replacement.