CH- Calcium Hardness

(CH) Calcium hardness indicates the amount of calcium in the water.
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.
Keeping the water saturated with calcium will protect pool surfaces from premature failure.
A plaster pool should have CH levels between 250 and 350.
Total hardness is the sum of calcium hardness and magnesium hardness.
Most test strips report TH instead of CH causing plaster degradation from low calcium.
To increase your CH level you can add calcium chloride.
Cal-hypo will also increase CH levels, for every 10ppm of free chlorine added to your pool you increase your CH by 7ppm.
To decrease you must remove water.