Spa FROG Cartridge keeps pH levels neutral . which reduces your chlorine or bromine use up to 50% and makes spa water feel silky soft and look crystal clear. Cartridges are easily adjusted to meet the sanitizing needs of virtually any size spa. Replace the cartridge every four months when you drain and refill your spa. SPA FROG Bromine Cartridges are for use in the SPA FROG Floating Systems . in which pre-filled mineral and bromine cartridges combine in a floating dispenser to give any spa crystal clear water with a silky feel. SPA FROG Bromine Cartridges snap easily into a reusable floating holder and when used in combination with the SPA FROG Cartridge . reduces the need for bromine by up to 50%. Replace the bromine cartridge every two to four weeks . depending on use.