10 inch replacement carbon block cartridge The black cartridge in the second chamber is a high grade carbon block element that will effectively remove any chemical impurities within the tap water and will polish the water. Carbon cartridges work in the same way as using activated carbon within an aquarium and will remove chlorine and chloramine which can damage the membrane and is not desirable in your aquarium. The levels of chlorine in the water in the UK are relatively low however we still suggest that these low cost filter elements should be changed every 6 months or earlier as necessary. Suitable for all reverse osmosis units