What is Calcium Hardness?
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 can be found in Calcium Saturation Index in Pool School and CSI and LSI.
Is Calcium Level Important in Plaster Pools?
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.
Is Calcium Level Important in Vinyl Pools?
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.
Is Calcium Level Important for Fiberglass Pools?
Calcium helps fiberglass pools resist staining and cobalt spotting. Over time, water with low calcium levels will tend to dissolve calcium out of fiberglass surfaces to some extent.
Is Calcium Level Important to Spas?
If you have a spa you might want to keep CH at at least 100ppm to 150ppm to reduce foaming.
What Calcium Level Should be Maintained in a Pool?
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.
How to Increase Calcium in Pool Water
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.
Beware of buying ice melters to increase CH.[1] Most of the cheaper ones are salt based. Some will increase your salt as well as your Calcium. Check the MSDS of non-pool products to find the ingredients.
How to Lower Calcium in Pool Water
You lower calcium by replacing water or using a reverse osmosis water treatment.
