Consequences of Adding Too Much
Muriatic acid is a solution of a strong acid called hydrochloric acid. Typically muriatic acid sold in hardware stores will be 30 to 35 percent hydrochloric acid. Adding too much hydrochloric acid can dramatically reduce the pH of your swimming pool water. Acidic pool water may corrode metal parts like pumps and filters, irritate or burn swimmers' eyes, and erode or etch pool plaster.
Consequences of Adding Too Little
If you don't add enough muriatic acid, on the other hand, the pH of your pool water may remain too high. Alkaline pool water can also irritate or sting swimmers' eyes and cause scale buildup on walls and surfaces; moreover, pool chlorine is not effective as a disinfectant at high pH. It's important to add enough muriatic acid to bring your pool water pH back into the optimal range.
Test the pH before and after adding muriatic acid to make sure you've adjusted it appropriately. The pH of your pool water should stay between 7.2 and 7.8, according to the North Carolina Division of Environmental Health; the optimal level is between 7.4 and 7.6. For a 1,000-gallon pool, add 1.5 oz. if the pH is between 7.8 and 8, 2.5 oz. if the pH is between 8 and 8.4, and 3 oz. if the pH is above 8.4. For a 5,000-gallon pool, add 8 oz. if the pH is between 7.8 and 8, 12 oz. if the pH is between 8 and 8.4, and 16 oz. if the pH is over 8.4. For a 10,000-gallon pool, add twice as much as you would for a 5,000-gallon pool to restore pH to the optimal level.