Spa Frog Mineral Cartridge refill for using in either the Spa Frog Floating System, or for users of the Spa Frog Inline System which is factory-installed in their spas. Product notes: Replace the mineral cartridge every 4 months when you drain and refill your spa. (The bromine cartridge will last 2-4 weeks depending on use). Refill cartridges are not for the Spa Frog In-Filter stick, which is not refillable.