Nature2 W20750 Spa/Hot Tub Mineral Sanitizer Cartridge Sticks (3 Pack) Designed for spas and hot tubs up to 500 gallons Destroys bacteria in spas to improve the look, feel and smell of your water Complete spa sanitizing system when used with lithium or non-chlorine spa shock Provides continuous sanitation using small amounts of dissolved minerals Slips into the center hole of spa cartridges and can last for up to four months