AQUA ONE CLEARVIEW CARTRIDGE CARBON - 300 47CThe Aqua One Carbon Cartridge is the replacement for the Clearview 300 external filters. Carbon Cartridges should be replaced every 6-8 weeks. They are easy to install and come in a pack of two. Always remember when cleaning the cartridge to use water from your fish tank, not tap water. This is to ensure no beneficial bacteria that breaks down fish waste in your aquarium is killed. Part Number: 25047C