Manage calcium hardness to avoid both scaling and etching

Steps

1. Test calcium hardness (as CaCO3) and compare it to the target operating band; commercial-pool guidance commonly cites roughly 200–400 ppm as preferred, with a lower ideal floor around 100–200 ppm for standard pools, though the exact ceiling has varied across MAHC editions — confirm the current number adopted locally rather than assuming one figure.
2. If hardness is low (contributing to a negative/etching LSI), raise it with calcium chloride in measured doses, retesting before adding more.
3. If hardness is high (contributing to a positive/scaling LSI), the only reliable fix is dilution (partial drain and refill with lower-hardness water) — hardness cannot be chemically 'lowered' in place.
4. For active scaling, brush and use a scale-appropriate cleaner on affected surfaces/equipment (e.g., heat exchangers, tile line) after bringing LSI back into a balanced range, since scale will keep forming if the water chemistry driving it isn't corrected first.
5. For active etching/surface erosion (rough, pitted plaster), correct the whole LSI picture (raise pH, alkalinity, and/or hardness as needed) promptly, since continued aggressive water will progressively damage plaster, grout, and metal fittings.