Why calcium matters
Water low in calcium tends to dissolve calcium from plaster, grout and pebble finishes (etching), while very high calcium with high pH can deposit scale on tiles, heaters and salt cells. Vinyl and fiberglass pools care less about calcium, though many fiberglass manufacturers recommend a minimum. Use the saturation index calculator to check the balance of calcium, pH, alkalinity and temperature together.
Adding calcium chloride
Dissolve it in a bucket of pool water first; it gets hot. Add with the pump running. Calcium chloride is the same compound sold for de-icing, but pool-grade products avoid additives.
Lowering hardness
Calcium only leaves with water, so the fix is a partial drain and refill with softer water. Ask a local professional before draining an in-ground pool: groundwater can lift an empty shell, and vinyl liners can shrink.
Not professional advice. Results are estimates for residential pools and hot tubs. Read and follow product labels: add chemicals separately, to water (never water to chemicals), with the pump running, and retest before adding more. Commercial and public pools must follow local health codes.