FROG Serene Cartridge Kit contains 3 bromine cartridges and 1 mineral cartridge. TheFloating Sanitizing System creates Fresh Mineral Water in your hot tub by killing bacteria two ways - with minerals and a low level of bromine. FROG Serene keeps your hot tub clean while eliminating the work and guesswork. Instead of spooning in chlorine daily, you simply replace a pre-filled bromine cartridge about once a month. With up to 50% less bromine*, you'll experience softer feeling water that's easier on skin, swimsuits and spa surfaces. Cartridges are easily adjusted to meet the sanitizing needs of hot tubs up to 600 gallons. The system comes with a start-up shock called FROG Jump Start and an introductory bottle of FROG Test Strips. Keep the green holder while replacing the blue mineral cartridge every 4 months (or when you drain and refill your hot tub) and the yellow bromine cartridge every 2-4 weeks depending on use. *Compared to the minimum ANSI recommended bromine level of 2.0 ppm for a hot tub.