There are so many different replacement cartridges it could make your head spin trying to locate the proper one. We are in the process of updating this area of the site in order to make it more user friendly. In the mean time please try the Live Chat on the left of the screen or give us a call at 888.799.7665. If you choose to contact us for help please have the following information ready: Name of Hot Tub Manufacturer, O.D. (outside diamter of cartridge), and Length of the cartridge.