If the LSI is not balanced, the pool surface is at risk of permanent damage. If your pool pro follows basic range chemistry, your water can still be out of balance and etch your pool's surface. For example, in the winter if your pool has a p.H. of 7.5, total alkalinity of 90, water temperature of 55F, calcium hardness of 300 ppm, cyanuric acid of 40ppm, and TDS of 1000, your water has an LSI of -0.34. This is deeply aggressive water. All of these numbers are within recommended ranges in classic pool chemistry textbooks, but the old way of “range chemistry” can be wrong. The life of your pool plaster surface will be greatly reduced if LSI is chronically low, as the water will look to find equilibrium of calcium carbonate and pull it from the plaster of your pool causing permanent etching. On the opposite end, if LSI is chronically high, your water will slowly deposit calcium all around your pool. This is the lesser of the two evils, as it is not permanent and can be chelated back into solution with treatment. Science!