Do I need to add calcium to my pool if my fill water is naturally soft?

Yes, you have to raise calcium hardness manually because soft water is aggressive and will eat at pool plaster, grout, or your heater. Target 200–400 ppm calcium hardness using a test kit. Add calcium chloride (hardness increaser) at about 1 pound per 10,000 gallons to raise hardness by 10 ppm, dissolving it in a bucket of pool water first and pouring near a return jet with the pump running. Straight calcium chloride works fine—skip expensive blends. Check calcium hardness a couple times a season or after heavy rain or refill.