FROG Serene in-Line Cartridges for Hot Tubs That Have Built in Cartridge Holders, FROG Serene Mineral Cartridge + 3 Bromine Cartridges (200 Grams)Brand: FROG Features: This FROG Serene In Line Cartridge Replacement Bundle gives you weeks and weeks of ahhmazing hot tub water, for use in FROG Serene In Line System. The Bromine and Mineral Cartridges fit in the Cartridge Holder installed on select Marquis Spas, Artesian, Caldera Spas and Hot Spring Spas FROG Serene is a complete sanitizing system that eliminates organic contaminants 2 ways with a low level of bromine and FROG Sanitizing Minerals