Aqua Kleen AK-4029 Unicel Replacement Filter Cartridge for Swimming Pool and Spa 25 sq. ft., 5 3/16" OD, Open Hole: 1 1/16" Top, Open Hole: 1 5/8" Bottom, 11 7/8" Long, AK-4029 Exact Fit for Unicel: C-5626 Exact Fit for Pleatco: PPM25-4 Exact Fit for Filbur: FC-3626 Guaranteed Perfect Fit - Extremely Durable - Wash & Reuse - Most Energy Efficient - Withstands High Pressures & Temperatures