SpaGuard Calcium Hardness Increaser (12oz) (3 Pack) add 1tbs. per 100 gallons to increase your hot tub calcium hardness 25ppm. Prevents corrosion of equipment and etching of plaster due to low calcium hardness. Product Information: Granular calcium chloride 1 tablespoon per 100 gallons increases calcium hardness 25ppm Prevents corrosion of equipment due to low calcium hardness Prevents etching of plaster due to low calcium hardness Directions of use: 1. Apply appropriate amount of product as measured by an accurate testing method 2. Do not pre-dissolve 3. Brush up any granules that do not immediately dissolve 4. Operate circulation system for 30 minutes after application.