Use only with hot tubs that use FROG @ease products. FROG @ease Test Strips are specially designed to precisely measure chlorine levels – other test strips may indicate chlorine levels are too low, and cause confusion. FROG Test Strips have a single color “Out Indicator” that shows when the SmartChlor Cartridge needs to be replaced; they also show Total Alkalinity, pH and Total Hardness levels. Each bottle contains 50 test strips. Only FROG Test Strips help you maintain the low level of Free Chlorine needed with FROG @ease hot tubs. *Compared to the minimum ANSI recommended chlorine level of 2.0 ppm. for a hot tub.