How to Lower Pool pH (and Keep It Down)
By The Aqualitx team at STEMania · Updated
If your test shows pH above 7.8, your chlorine is working at a fraction of its strength and the water is starting to favor scale. The fix is acid — but how much, which kind, and how fast all matter. This guide walks through each.
What pH should a pool be?
Aim for 7.2 to 7.8, with 7.4 as the everyday target. Below 7.2 the water gets corrosive to metal, plaster and eyes. Above 7.8, free chlorine shifts into its slow form (hypochlorite ion), so the same chlorine reading kills far less, and calcium starts dropping out as scale on tile, in the filter and inside the heater.
Why pH keeps rising
Rising pH is the normal state of most pools, not a sign something is wrong. The main drivers:
- Aeration. Waterfalls, spa spillovers, jets and swimmers drive carbon dioxide out of the water, and pH climbs as it leaves. The higher your alkalinity, the faster this happens.
- Salt cells. A salt chlorine generator raises pH as it makes chlorine.
- Fresh plaster. New plaster pushes pH up for months.
Trichlor tabs push the other way — they're acidic — which is why tab-fed pools often drift *down* instead.
Which acid to use
- Muriatic acid (hydrochloric acid, usually 31.45%) is the cheapest and most common. It fumes, so pour it low and slow, outdoors, upwind.
- Dry acid (sodium bisulfate) is a granular powder that's easier to store and handle. It costs more per dose and adds sulfates over time.
Both do the same job. Pick the one you'll handle safely.
How much acid to add
As a starting point, about 6 oz of dry acid per 10,000 gallons lowers pH by roughly 0.2. Muriatic acid works out similarly by volume. Real pools vary a lot with alkalinity: high-alkalinity water resists the change and needs more acid for the same drop.
For an exact number, run an acid demand test (the R-0015 reagent in kits like the Taylor K-2006). Each drop it takes to reach your target pH works out to about 25.6 fl oz of muriatic acid, or 24 oz of dry acid, per 10,000 gallons. The pH calculator does that math for your pool's volume.
How to add acid safely
- Run the pump, and keep it running for the next several hours.
- Wear eye protection and gloves. Never add water to acid — add acid to water.
- Pour slowly into the deep end, close to the surface, away from the skimmer and any metal fittings. Some owners pre-dilute muriatic in a bucket of pool water first.
- Keep swimmers out until it has circulated.
- Wait about 30 minutes, then re-test pH and alkalinity before adding more.
Never mix acid with chlorine in the same bucket, and don't add the two to the pool at the same time — together they release chlorine gas. Give the acid time to circulate first.
Acid lowers alkalinity too
Every dose of acid pulls total alkalinity down a few ppm along with pH. That's useful when alkalinity is high, but if you keep chasing a rising pH with acid week after week, alkalinity can slide below 80 ppm and pH starts swinging. If that happens, raise alkalinity back up — see how to raise pool alkalinity.
The longer-term fix for a pH that climbs every few days is often to run alkalinity at the low end of its range (around 80 ppm). Lower alkalinity means less carbon dioxide driving the drift.
The short version
- Target pH 7.4; act above 7.8.
- Use muriatic or dry acid, sized to your pool — an acid demand test makes it exact.
- Add no more than a quart of muriatic per 10,000 gallons at a time, pump running.
- Wait 30 minutes, re-test pH and alkalinity, then decide on more.