pH Decrease is used when the pH level in your hot tub is too high.
pH is a scale measurement of the acidity or alkalinity of the water. The scale runs from 0–14. 0 being extremely acidic and 14 is very alkaline. Neutral is at 7.
The pH of the human eye is about 7.4 so for comfortable swimming, it is best to have your pool water at the same level. If you drop too far below 7.4, the water is too acidic and stings your eyes. If the pH rises too far above 7.4 the alkalinity irritates your eyes.
7.2 – 7.6 is the range in which chlorine is most effective as a sanitizer. If you pH is too high or low than the chlorine will be ineffective at eliminating the bacteria in the water.