The Penn Plax Cascade 300 GPH Filter Cartridges will improve the quality of your aquarium water and keep your fish healthy. This replacement filter is designed to fit your Cascade 300 GPH hang-on power filter and will last 6 to 12 weeks under normal conditions. The filter replacements are great for freshwater aquarium filtration with a poly-fiber floss barrier for an extra layer of protection. Within 24 hours you will see a drastic difference in the water quality. Each filter cartridge is perfectly measured to fit into the Cascade 300 Hang on Filter tray without any gaps. This filter cartridge is suitable for both freshwater and marine environments. When used together; the Cascade 300 GPH Filter Model CPF5 and Cascade 300 filter cartridges provide maximum protection for your fish.