Freshwater AG+ Silver Ion Sanitizer is made for HotSpring Spas, by Nature 2 / Zodiac. Silver ions are one of nature's most powerful sanitizers are continuously introduced into your spa water to inhibit bacteria growth. Simply replace the cartridge every four months for on-going protection. For a chlorine free experience use SpaGuard Spa Shock to start the process of easy spa care. This cartridge is distributed for use in Watkins manufactured spas, such as Hot Spring, Tiger River, Hot Spot, and Solana models