Low calcium hardness in your pool can cause pool water to be corrosive, resulting in damage to pool surfaces and equipment. Test your water weekly to make sure your calcium hardness levels are between 200 and 500 ppm. If it's below 200 ppm, add HTH Calcium Hardness Up to raise calcium hardness levels and prevent corrosion of metal and plaster surfaces. HTH Calcium Hardness Up is great for all pool types including vinyl-lined pools and saltwater systems. Compatible with all swimming pools. Protects pool surfaces. Helps chlorine to work efficiently. Protects pool liners, metal and plastic surfaces as well as equipment from damage.