Use acid, and get the amount right
pH comes down with acid, almost always muriatic acid (sometimes sold as dry acid, sodium bisulfate). The trick is that alkalinity resists the change, so a pool with high alkalinity needs much more acid than one with low alkalinity to move pH the same amount. A flat rule like a cup per so many gallons is usually wrong.
Skip the guesswork and use the pH calculator, which uses your alkalinity to work out the actual acid dose to reach your target. Aim for a pH of 7.4 to 7.6.
Add it slowly and safely
Acid is corrosive, so handle it with care. Add it with the pump running so it mixes, pour it slowly into the deep end or around the perimeter, and never add it near the skimmer or mix it with other chemicals. Add no more than about a third of the calculated amount at a time, then retest before adding more.
It is far easier to add a little more acid than to fix an overshoot, which can crash pH and etch plaster. Patience here saves you a second correction.
Acid lowers alkalinity too
Muriatic acid lowers both pH and total alkalinity at the same time. That is useful when both are high, but it is worth knowing, because if your alkalinity is already low, dropping pH with acid can pull it too far down. Check your alkalinity with the alkalinity calculator and keep it in the 80 to 120 ppm range.
Why your pH keeps climbing
If you are always chasing high pH, the cause is usually high total alkalinity or aeration. Alkalinity pushes pH up, so an alkalinity that runs high will keep dragging pH up with it no matter how much acid you add. Salt systems, waterfalls, and jets also off-gas carbon dioxide, which raises pH.
The real fix for a pool that constantly drifts high is to bring total alkalinity down a little, so pH rises more slowly, rather than dosing acid every few days. Lowering alkalinity is the same acid, just aimed at the buffer.
Frequently asked questions
What lowers pH in a pool?
Acid. Muriatic acid is the standard choice, and dry acid (sodium bisulfate) is an alternative. The amount depends on your total alkalinity, which buffers pH, so use a calculator that accounts for it rather than a fixed dose. Aim for a pH of 7.4 to 7.6.
How much acid do I need to lower pool pH?
It depends on your pool volume, how far pH needs to drop, and your total alkalinity. There is no fixed amount per gallon because alkalinity resists the change. The pH calculator works out the muriatic acid dose from those numbers. Always add in stages and retest.
Will lowering pH also lower alkalinity?
Yes. Muriatic acid lowers both pH and total alkalinity together. That is helpful when both are high, but if your alkalinity is already low, be careful not to pull it down too far. Check alkalinity alongside pH and keep it in the 80 to 120 ppm range.
Why does my pool pH keep going up?
Usually high total alkalinity or aeration. Alkalinity drags pH upward, so a high alkalinity will keep raising pH no matter how much acid you add. Salt systems and water features off-gas carbon dioxide and raise pH too. The lasting fix is lowering alkalinity a little.
Can I lower pH without lowering alkalinity?
Not really with acid, since acid affects both. The usual approach is to lower both with acid, then aerate the water, which raises pH back up while leaving alkalinity down. That lets you target alkalinity without leaving pH too low.