"The Langlier Index of Saturation",states you need a calcium level of 150ppm - 300ppm. Soft water is aggressive and will cause pitting and etching of concrete, grout or plastered surfaces as it seeks to dissolve calcium into the water from whatever contact source.
However calcium's purpose is not to prevent the water from being corrosive (as that is what the pH is for), but rather to add a temporary level of protection to the surfaces that come into contact with corrosive water, particularly heaters, metal fittings and concrete pool surfaces. The way calcium does this is by leaving a thin calcium film on these surfaces, when the water balance of pH, alkalinity and calcium is ideal. If your pH does drop below 7 (ideal is 7.4-7.6) the corrosive water has to first etch the calcium film off before it can corrode the metal or concrete surfaces adding a degree of temporary protection to these vulnerable surfaces.
It should be noted that if the water balance is not right, for example the pH is to high, then calcium can cause excessive scaling. This scaly build up starts in the heater and in extreme condition leaves a chalky residue or scale at the water level and in the filters as well as causing cloudy water. When this scale builds up excessively on the heater or in the heat exchangers, then the calcium acts as an insulator causing the heater to be inefficient or even worse overheat and burn out on an electric heater, or cause a heat exchanger meltdown on a gas heater. There are chemicals that prevent or reduce scaling when your pH goes up or your calcium levels are to high, but at the same time it reduces the beneficial effects of the calcium hardness in addition many brands of water softeners promote foaming with extended use, making you wonder why you put calcium in, in the first place.
When conditioning your water it is always best to adjust alkalinity first, calcium hardness second and finally the pH as the pH is affected by the amounts of calcium and alkalinity in the water.
To Reduce Calcium Hardness:
Dilute pool or spa water (partially drain and refill) Tri-Sodium Phosphate (TSP) could be used to precipitate calcium out of the water, but it is not recommended due to it causes algae bloom (the phosphates are fertilizer for algae).
To increase the Calcium Hardness
Use Calcium Chloride (at 77%). Note: Calcium Carbonate is also commonly used.
If high calcium levels is prevalent in your area a source of better water is needed
Soft water from a water conditioner will give you low calcium level water but it will add salt to the water raise the TDS, may not allow the sanitizers to work correctly and can cause cloudy water