Unicel Replacement Filter Cartridge 50 sf Cal Spa/Martec | C-4305Unicel C 4305 Replacement Spa Filter Cartridge 50 Sq Ft (Open Top & Bottom, 2 1 8" ID) The Unicel C 4305 is a premium 50 sq ft pleated replacement filter cartridge used in many hot tub and spa filtration systems that take a 4 15 16" diameter cartridge with open ends. Replacing a worn or torn cartridge helps restore circulation, improve water clarity, and reduce strain on the spa pump and heater. Key Features 50 sq ft filtration area for strong debris