When we speak of scale, we are talking about calcium carbonate which has come out of solution and deposited itself on surfaces. It is a combination of carbonate ions, a part of total alkalinity and calcium, and a part of the Calcium Hardness level. The test for Calcium Hardness is a measure of how "hard" or "soft" the water is testing. "Hard" water can have high levels of calcium and magnesium. If these levels are too high the water becomes saturated and will throw off excess particles out of solution which then seeks to deposit themselves on almost any surface inside the spa. This is calcium carbonate scale; a "white-ish," crystallized rough spot. If the levels are too low, the water is under-saturated. If under-saturated, the water will become aggressive as it attempts to obtain the calcium it needs. Such "soft-water" will actually corrode surfaces inside the spa which contain calcium and other minerals to maintain its hardness demand. Recommended range for calcium hardness is 175-300 PPM. Calcium Hardness levels should be tested weekly.