Spa Frog combines the use of natural minerals with a small, measured amount of bromine to naturally purify your spa water and create a difference you can really feel. Spa Frog is compatible with most other hot tub water care products on the market, including the full range of SpaGuard products. The Spa Frog cartridges are easily adjusted to meet the sanitising needs of virtually any size spa. Adjustment is performed by turning a dial on the side of each spa. You should replace the Spa Frog mineral cartridge approximately every 3-4 months when you drain and refill your spa. you can tell when the mineral cartridge is empty by shaking it.