The Nature II spa purifier is an easy alternative to chlorine and bromine, which fits inside your filter cartridge. Its natural mineral purifier system lasts a full 4 months. Odorless if used with Spa Care Shock Oxidizer (potassium monopersulfate). Makes your spa so easy to manage. Assures an odor-free hot tub, while significantly improving the look & feel of the water. Simplifies water balance since it releases its sanitizer automatically.