Calcium hardness (CH) reflects the concentration of dissolved calcium in pool water. Water with too little calcium hardness can become aggressive toward plaster, grout, and metal fittings, drawing calcium out of these surfaces to compensate — a slow but real cause of surface degradation over time. Water with excessively high calcium hardness is prone to scale formation on surfaces and inside equipment, including filters and heating elements, which can reduce equipment efficiency.
The appropriate target range depends on the pool’s surface finish and, to some extent, local fill-water hardness, so it’s best checked against your test kit or a pool water-chemistry reference specific to your surface type rather than a single number assumed to apply everywhere.