Aqua Kleen AK-3003 Unicel Replacement Filter Cartridge for Swimming Pool and Spa 8 sq. ft., 3 1/4" OD, open hole: 1 1/16" top, open hole: 1 1/16" bottom, 9 3/4" long, ak-2001 Exact fit for unicel: c-3608 Exact fit for pleatco: ps9-4 Exact fit for filbur: fc-3076 Guaranteed perfect fit - extremely durable - wash & Reuse - most energy efficient - withstands high pressures & temperatures