Soda ash

Soda ash is the standard fix when pH runs low. It raises pH with little change to alkalinity. If you want to raise alkalinity instead, use baking soda.

Add small amounts, with the pump running, and retest after a few hours. Too much clouds the water and can leave scale. Wear gloves and eye protection, because the dust irritates skin and lungs. Keep it dry and away from acids and chlorine. If your pH keeps falling, look for the cause, such as trichlor use or low alkalinity.