CSI and LSI are two similar measures of calcium saturation in the water. LSI is a simplified version designed for manual calculation, while CSI is a more precise version that requires computer assistance to calculate. When the saturation index is too negative the water will try to dissolve calcium out of pool surfaces. A low saturation index is a problem for pools with plaster/pebble/quartz/tile surfaces. When the saturation index is too positive the water will tend to deposit calcium on the pool surfaces. A high saturation index is a problem for any kind of pool.