AquaPro NSF Certified (CTO) carbon block filters are manufactured with high-purity coconut shell-activated carbon and are available in a wide range of lengths, diameters, and micron ratings. AB series provide an exceptional cost-to-performance value. With high chlorine reduction, great dirt-holding capacity, and greatly reduced carbon fines, you will soon make the AquaPro series your carbon block of choice. AquaPro CTO cartridge filters remove impurities from the water, thus preventing the RO membrane from getting choked. CTO cartridge is made to the highest specification. CTO filter is used as Filters & Reverse Osmosis water filters. This CTO replacement filter fits most standard Filtration housings and most Reverse Osmosis systems.