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How to Lower Alkalinity in a Pool
Acid lowers alkalinity; aeration brings pH back up. Two answers give the acid.
1 of 2: your gallons
Your dose appears here after the last question.
- 120 ppm
- lower it above this
- Several days
- split the acid into doses
- 7.2
- let pH fall to here, then aerate
To lower alkalinity in a pool, use muriatic acid, which pulls the pH down, followed by aeration, which lifts the pH again. Do that back and forth for a few days until the alkalinity settles where you want it.
- Where you want alkalinity to end up: somewhere between 80 and 120 ppm (parts per million)
- To trim alkalinity by 10 ppm in a 10,000-gallon pool, plan on 26 fl oz of muriatic acid
- How far to let pH drop: down to roughly 7.2, then aerate
- How long it takes: a few days, splitting the acid into separate doses
Total alkalinity measures how hard the water pushes back against pH changes. If it climbs too high, your pH will keep creeping upward. Both readings use ppm, or parts per million. That's simply how much of something is dissolved in the water, and it's the figure your test kit or strips reports.
Pool alkalinity too high
Once you're past 120 ppm, it's time to act. Two things usually cause it: the water you fill with already carries a lot of alkalinity, or the pool got more baking soda or soda ash than it needed at some point.
Muriatic acid is the strong acid pools use for exactly this. All the amounts here assume the 31.45% strength. The table shows the total amount of acid to pour, given in US fluid ounces. If the drop you need is a big one, break the total into doses across several days instead of adding it all in one go.
| Volume of water | Drop of 10 ppm |
|---|---|
| 5,000 gal | 13 oz |
| 10,000 gal | 26 oz |
Say your pool holds 20,000 gallons and needs a 10 ppm drop. That works out to double the 10,000-gallon dose, spread across several days.
Adding the acid safely
Some simple habits make this safer and help the acid spread out:
- Keep the pump going as you pour, and aim your pour near a return jet so the acid circulates everywhere.
- Put in just one chemical at a time.
- Pour the acid into the water, not water onto acid.
- Keep acid and chlorine apart.
- Retest the pH half an hour after each dose.
How to lower alkalinity without lowering pH
There's no chemical that drops alkalinity by itself and leaves pH untouched. But the acid-and-aerate cycle gets you there, since the alkalinity stays low while the pH climbs back. Aeration just means working air into the water. Aim your return jets at the surface, or run a waterfall, and that does it.
- 1. Dose with acid until the pH drops to roughly 7.2. Both readings go down together when you add acid.
- 2. Aerate until the pH climbs back to about 7.6. The pH rises, but the alkalinity holds where the acid left it.
- 3. Test both numbers, then run the cycle again every day or two.
- 4. Quit once alkalinity hits 80 to 90 ppm. You want pH between 7.4 and 7.6, and 7.2 to 7.8 is still fine.
Questions people ask
How can I bring alkalinity down quickly?
Only acid will bring alkalinity down. Even so, expect it to take days, since every dose needs aeration and a new test after it.
Is it possible to drop alkalinity and pH together?
Yes. If both numbers are high, acid alone takes care of them. Hold off on aerating until pH lands between 7.4 and 7.6.
Is 40 ppm a healthy alkalinity level?
No, that's well under the recommended 80 to 120 ppm. Baking soda will lift it: 1 lb 6 oz per 10,000 gallons adds roughly 10 ppm. So a climb from 40 up to 80 ppm calls for about 5 lb 8 oz per 10,000 gallons.
Does pool size change the acid amount?
Yes, the dose tracks your gallons. Double up at 20,000 gallons, and use half at 5,000.