Use a water-testing kit to measure the calcium hardness (how “hard” or “soft” the water is). The proper calcium hardness is between 200 and 400 parts per million (ppm).

Following package directions, add calcium carbonate dihydrate to raise calcium hardness; add sodium hexametaphosphate to lower it. Carefully pour the chemical mixture into the pool at various spots a foot or two away from the sides of the pool.