What can be added to increase the acidity in a pH range above 7?

Muriatic acid is the appropriate choice for increasing acidity in a pH range above 7. It is a diluted form of hydrochloric acid and is commonly used in swimming pool maintenance to lower the pH level of water. When the pH is above the ideal range, which is typically between 7.2 and 7.8, adding muriatic acid effectively introduces more hydrogen ions into the water, thereby increasing its acidity and correcting the pH level.

In contrast, calcium hydroxide and soda ash are substances used to raise pH levels. Calcium hydroxide, often referred to as lime, increases alkalinity and can raise the pH if it is low. Soda ash (sodium carbonate) is similarly used to elevate pH levels and also increases water alkalinity. Bleach, or sodium hypochlorite, is primarily used as a disinfectant and can affect pH levels slightly, but it is not intended for adjusting acidity. Thus, muriatic acid is specifically effective for the purpose of reducing pH and increasing the acidity of pool water.