HTH(TM) calcium hardness increaser, 4 lbs., is used to raise the calcium hardness of swimming pool water. Low calcium hardness may cause the pool water to be corrosive, resulting in damage to pool surfaces and equipment. Plaster finish pools may be etched or stained if the calcium hardness of the pool water is not properly adjusted. The calcium hardness of chlorinated pool water should be maintained in the range of 200-500 ppm.