FROG @ease Replacement SmartChlor Chlorine Cartridge - 3 Pack (01-14-3258) Low-chlorine spa sanitizing cartridge maintains a consistent 0.5  1.0 ppm free chlorine level at all times Shock only once a month and use up to 75% less chlorine Floats in spa water and flips over when empty, indicating the SmartChlor cartridge needs replacing Lasts 3-4 weeks in spas up to 600 gallons Designed for use with Frog @ease Mineral System - Mineral Cartridge not included