The quick answer: if your pool's pH reads above about 7.8, add muriatic acid or dry acid to bring it back to about 7.4–7.6.
How much acid you need depends on your volume, how far you're going and your total alkalinity (TA), so run the calculator rather than trusting a one-size chart. Then deal with the cause: in most pools, pH keeps rising because TA is higher than it needs to be.
What's "too high"?
| pH reading | What it means |
|---|---|
| 7.2–7.8 | Acceptable |
| 7.4–7.6 | Ideal for most pools |
| 7.8–8.0 | High. Correct it soon. Scale risk rises, especially with high calcium. |
| Above 8.0 | Too high. Correct it now, before anyone swims: the Model Aquatic Health Code treats public pools above 8.0 as needing immediate correction. Most phenol red test kits can't read above about 8.2, so the real value may be higher. Above 8.5 it is not safe to swim. |
If you just shocked the pool and pH reads purple or oddly high, retest when FC is back to normal levels. Very high chlorine interferes with the phenol red pH test.
Symptoms of high pH
- Cloudy or dull water, often with a white or grey haze.
- Scale: white crust at the waterline, rough plaster, and scale in heater or salt cell plates.
- Itchy eyes and skin. Comfort is best close to the pH of tears (around 7.4–7.6).
- Chlorine feels less effective. This is real, but smaller than old charts suggest when you use stabilizer (CYA). High pH is rarely the main reason a pool turns green; low FC relative to CYA usually is. See the FC/CYA chart.
Why pool pH rises
Pool water holds dissolved carbon dioxide (CO₂), mostly from the bicarbonate that makes up your TA. That CO₂ constantly escapes into the air, and every bit that leaves pushes pH up. Anything that speeds up the escape makes pH rise faster:
- High total alkalinity. More bicarbonate means more CO₂ to lose. This is the number one cause of "my pH goes up every few days".
- Aeration: waterfalls, spillover spas, deck jets, fountains, return jets pointed up at the surface, and heavy swimming and splashing.
- Salt water generators. The cell's hydrogen bubbles aerate the water, so SWG pools almost always drift up.
- New plaster or pebble finishes. Fresh cement releases lime and raises pH for months.
- Chemicals: soda ash, borax and cal-hypo (which is alkaline) all push pH up. Liquid chlorine raises pH a little when added, but most of that reverses as the chlorine is used up, so its net long-term effect is small.
- High fill-water pH or TA, if you top up a lot.
How to lower pH
- Test pH and TA. You need TA to calculate the acid dose.
- Calculate the acid amount with the calculator. Enter the strength on your label: 31.45% or 14.5% muriatic acid, or dry acid.
- Add it safely:
- Pump running.
- Pour slowly into the deep end, away from the skimmer.
- Never at the same time or spot as chlorine.
- Wear goggles and gloves.
- Large dose? Split it. Add the first part, circulate for 30 minutes, and retest.
- Retest after 30 minutes and adjust.
Full steps, strengths and safety notes are in how much muriatic acid to lower pH.
How to keep pH down
Adding acid fixes today. Stopping the climb fixes the summer.
- Lower TA if it's high. Many pools run happily with TA around 60–80 ppm, and SWG and plaster pools often at the lower end. The right number depends on your surface, chlorine source and CSI. Lower TA means slower pH rise. Use the acid-and-aeration method in pool alkalinity too high.
- Add borates (about 50 ppm). Borates add pH buffering that resists upward drift. They're especially useful in SWG pools and pools with water features.
- Reduce aeration you don't need. Turn off waterfalls and spillovers when no one is enjoying them. Point returns slightly down.
- Switch from cal-hypo to liquid chlorine if you're using cal-hypo daily.
- Watch the trend, not one reading. Logging pH every test tells you how many days it takes to drift from 7.5 to 7.8. That's your acid schedule, and it shows whether lowering TA worked.
Check your water balance (CSI)
High pH is one input to the Calcite Saturation Index (CSI), which predicts whether water will deposit scale (positive CSI) or dissolve plaster (negative CSI). The other inputs are calcium hardness, TA, temperature, CYA and salt.
A pool at pH 7.9 with low calcium may be perfectly balanced. The same pH with high calcium and warm water can scale your heater. The CSI calculator puts it together. Aim for roughly −0.3 to +0.3.
Common mistakes
- Using a fixed acid chart. At low TA, it overshoots and pH crashes below 7.0.
- Chasing pH daily without fixing TA. You'll use far more acid in total.
- Adding acid and chlorine together. Wait 30 minutes between them.
- Reading pH right after shocking. Wait for FC to come down.
- Relying on strips for pH decisions. A drop test (phenol red) is easier to read accurately.
Track it in Pool Calc
Pool Calc calculates the acid dose from your actual TA, splits big doses and logs every reading. You'll see your pH drift on a chart and get a reminder before it goes high. CSI is included.
One pool? Every feature, free. No account, no ads, and iCloud backup is included.
Safety
- Keep chemicals out of reach of children and pets, and keep them out of the water until it is back in range.
- Never mix chemicals. Never add acid and chlorine together: mixing them releases toxic chlorine gas.
- Add chemicals to water, never water to chemicals.
- Store chemicals sealed, cool and dry, apart from each other.
- Read and follow the product label. It always comes first.