Calcium Hardness is a measure of the amount of calcium and magnesium salts found in the water.
Your spa or hot tub water needs to have a Calcium Hardness level of between 100ppm and 200ppm.
Low Calcium Hardness (very soft water) may cause spa or hot tub water to be corrosive which may result in staining, etching or distorting of all types of spa and hot tub surfaces.
Surfaces etched by low Calcium Hardness will become more abrasive and uncomfortable for bathers.
Rough surfaces also increase problems with algae and makes cleaning more difficult.