Pool dry acid, also known as sodium bisulfate, is a chemical compound commonly used in swimming pools to lower the pH and alkalinity levels of the water. It comes in a granular or powder form and is typically added directly to the pool water to help balance the chemical levels and prevent issues such as scaling and cloudy water. It is important to follow manufacturer recommendations and safety precautions when using pool dry acid to avoid over-treating the water or causing harm to yourself or the pool equipment.