Pleatco replacement swimming pool/spa pool filter cartridge # PWK45N replaces Unicel C-8465, Filbur FC-3960, and Watkins Hot Spring Spas swimming pool/spa filter cartridges. Pleatco premium filter cartridges employ Free Flow Cores, specifically engineered to allow for the even flow of water through the entire cartridge which increases filtration (best efficiency), uses greater media surface to inhibit the growth of bacteria mold & algae (best water quality), and makes it easier for the pump to pull more water through the cartridge (less wear & tear and expense).