Clorox Pool&Spa Calcium Hardness Increaser raises low Total Hardness levels in your swimming pool that can cause damage to your pool surface. Ideal for plaster pools & soft water areas. Use it to raise Total Hardness levels in your pool that are below 175 ppm. Increasing Total Hardness in your pool prevents surface damage like corrosion, staining, and etching. 5 lbs. per 10, 000 gallons will raise increase your Total Hardness levels by 50 ppm.