Use Castle Hot Tubs pH Minus to reduce the pH level in your hot tub, spa or swimming pool. pH is a measure of how acidic or how basic water is. The pH range goes from 0 to 14, with 7 being neutral. A pH of less than 7 indicates acidity, a pH of greater than 7 indicates a base. The pH of water is a very important measurement concerning the water quality of your hot tub / spa or swimming pool. A high PH level in a pool or hot tub water can cause limescale build up, which can damage working parts including pumps, heaters and seals. To prevent this issue, bring the PH to between 7.2 and 7.4 on your test kit/strips.