what chemical increases water hardness in swimming pools
Leslie's 14427 Hardness Plus, 25 lbs. Leslie's 14428 Hardness Plus, 45 lbs. Keep your calcium hardness at proper levels at all times to help protect your pool, water, and pool equipment. The calcium hardness level in a pool does not increase rapidly, but it is good practice to check the calcium hardness every month.
It can also be accomplished by dilution (adding water to the pool which has a lower calcium hardness content). Levels which are too low require the addition of calcium chloride. Recommended range for calcium hardness is 200-400ppm. Calcium Hardness levels should be tested weekly with fresh reagents or strips.