The FROG Serene Mineral Replacement Cartridge is a quick and easy sanitizing solution for hot tubs. Made by Frog, the mineral sanitizer comes in granular form and is designed to float in the water. This blue cartridge is specifically for fresh water hot tubs and is easy to use, providing a convenient way to keep your hot tub clean and sanitized. With the FROG Serene Mineral Replacement Cartridge, you can enjoy a relaxing and hygienic hot tub experience.