Calcium hardness (CH) is the direct measure of the amount of calcium ions (Ca2+) in your pool water. Calcium hardness is different from total hardness (TH) or general hardness (GH) as those two parameters include magnesium hardness as well. Magnesium ion concentration is mostly irrelevant for pool water and does not need to be included in hardness measurements. CH, along with pH and TA, is one of the parameters needed to calculate the calcite saturation index or CSI. The CSI tells you the degree to which your water is saturated with calcium carbonate. More details about the CSI and it’s calculations can be found in the TFP Forum.
Over time, water with low calcium levels will tend to dissolve calcium out of plaster, pebble, tile, stone, concrete, and to some extent fiberglass surfaces. You can prevent this from happening by keeping the water properly saturated with calcium. In a vinyl liner pool there is no need for calcium, though high levels can still cause problems, mostly from the scaling of calcium carbonate. A plaster pool without a SWG should have CH levels between 250ppm and 350ppm if possible. With a SWG, CH should be kept between 350ppm to 450ppm. Calcium helps fiberglass pools resist staining and cobalt spotting. If you have a spa you might want to keep CH at at least 100ppm to 150ppm to reduce foaming.
You increase CH with calcium chloride, sold as a deicer and by pool stores, or calcium chloride dihydrate, sold by pools stores for increasing calcium. In some parts of the country (the southwest for example), high water hardness leads to CH naturally increasing in pools when there is more evaporation than precipitation. You lower calcium by replacing water or using a reverse osmosis water treatment.
