A number of different factors can contribute to a high pH level in a swimming pool. The most common cause of a high pH is high Total Alkalinity (TA). High TA can arise when the pool has a large amount of dissolved alkaline substances, such as carbonates.
Another cause of high pH is the use of pool chemicals in incorrect amounts. Chemicals like sodium carbonate, used to raise pH when the pool is overly acidic, can be overused, causing a high pH. Calcium hypochlorite, an alkaline substance used to add more chlorine to the pool, also can cause a high pH. When a pool has high pH, it can be lowered through the addition of chemicals like muriatic acid or sodium bisulphate.