4) Unicel C-8465 Hot Springs Spas Watkins 65 Sq Ft Replacement Filter Cartridges 4 New Unicel C 8465 65 SQ. FT. Watkins/Hot Springs Spas Replacement Cartridge Filters Outside Diameter: 8 1/2" Length: 10 1/2" Top end cap: Open, 3" inner diameter Bottom end cap: Open, 3" inner diameter