Aquatop SP913UV-RCI, Replacement Cartridge Insert for the SP9-UV and SP13-UV, 2 pack. The SP-UV 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. FEATURES: Original replacement filter cartridge for the SP9-UV and SP13-UV Provides mechanical, chemical & biological filtration Packed with Aquatop's premium activated carbon For best results, replace monthly 2 cartridges in each pack