Muriatic Acid is the most common chemical used to lower pH in the commercial swimming pool market. Muriatic Acid will also lower Total Alkalinity levels so care must be taken to properly balance the water chemistry.
Maintaining a consistently clean and safe swimming pool requires constant attention to the water's chemistry. Substances such as muriatic acid is used to lower the pH factor, which affects every other chemical balance. This is a measure of the hydrogen ion concentration. The scale runs from 0 (strong acid) to 14 (strong base) with 7 being a neutral pH. The optimum range for pools is 7.4 to 7.6.
Muriatic Acid MSDS Sheet