How to lower pH in a pool

High pH is the most common balance problem in pools, and it matters more than it looks. When pH climbs, chlorine gets weaker, and the water starts to scale.

The fix is acid, added carefully and in small doses.

Why high pH is a problem

Chlorine in water splits into a strong form and a weak form, and pH decides the mix. CDC's pool code gives the numbers. At pH 7.5, about half of your chlorine is in the strong form. At pH 8.0 it's only about a quarter. The effect is biggest in water without stabilizer; a published analysis by S.R. Pickens found stabilizer makes pH matter much less. Either way, keeping pH in range helps your chlorine do its job.

High pH also lets calcium drop out as scale. Shasta County notes that above pH 8.0, "scaling ... and cloudy water may result."

The CDC says to keep pH between 7.0 and 7.8. The Pool & Hot Tub Alliance's ideal is 7.4 to 7.6.

How much acid to add

Health departments use a simple rule of thumb. Lucas County, Ohio and Yakima County, Washington give the same figures: per 10,000 gallons, each 0.2 drop in pH takes about 12 fl oz of muriatic acid or about 1 lb of dry acid.

Here's what that looks like for common pool sizes, lowering pH from 7.8 to 7.6:

Pool size	Muriatic acid	Dry acid
10,000 gal	12 fl oz	16 oz
15,000 gal	18 fl oz (2.3 cups)	24 oz
20,000 gal	24 fl oz (3 cups)	2 lb
25,000 gal	30 fl oz (3.8 cups)	2 lb 8 oz

These amounts assume full-strength muriatic acid (the HASA label is 31.45%). If your jug lists a lower %, you'll need more, so check the % on the jug and follow its label.

Treat the table as a starting point. Both health departments say the real amount depends on your alkalinity: the higher it is, the more acid you'll need to move pH. That's why the safe approach is to add about half, let the pump run for an hour, retest and then add more. For your exact pool, use the pH calculator.

How to add acid safely

Wear safety goggles and gloves; the CDC lists both. Always add acid to water, never water to acid. Turn the pump on first. The HASA muriatic acid label says to pour slowly into the deep end, at least a foot from the wall, with the pump running during and for at least an hour after.

Never add acid and chlorine at the same time, and never let them touch, even in a bucket or in storage. The CDC's warning is blunt: "NEVER mix ... especially chlorine products with acid." Massachusetts warns that mixing pool chemicals can release toxic chlorine gas. Add one, let it circulate, then add the other.

When in doubt, use less. You can always add more after you retest. The HASA label also limits how much you add in one day, so read yours.

Why pH keeps going up

If you're lowering pH every few days, something is pushing it back up. The usual causes:

1. High alkalinity. The Pool & Hot Tub Alliance says that when carbonate alkalinity is too high, pH drifts upward as carbon dioxide escapes. Lower alkalinity into range and pH settles.
2. Splashing, waterfalls, spa spillovers and fountains. Aeration drives off carbon dioxide, which raises pH. Orenda and the Pool & Hot Tub Alliance both describe this.
3. Salt chlorine generators. The Pool & Hot Tub Alliance lists them as a cause of rising pH.
4. Liquid chlorine. It's very alkaline when it goes in (pH around 13, according to the Pool & Hot Tub Alliance) and can push pH up.

The first one is the one you can actually fix. Test alkalinity, and if it's high, use the alkalinity calculator to find how much acid it takes to bring it down.

Don't overshoot

pH below 7.0 is its own problem. Shasta County and San Francisco both warn that low pH irritates eyes and corrodes metal and plaster. If you overshoot, raise it with soda ash; the pH calculator handles that direction too.

Keep a log of how much acid you add each week on the test log. Once you know your pool's pattern, you can make smaller, steadier corrections.