Muriatic acid is hydrochloric acid, typically sold at 31.45% — 20° Baumé. It lowers pH, and it lowers total alkalinity at the same time, which is why the two are one adjustment rather than two.
The dose
For 31.45% muriatic acid, 10,000 gallons, to move pH down by the amounts shown, at a total alkalinity of 100 ppm:
| pH from → to | Acid |
|---|---|
| 7.8 → 7.5 | 10 fl oz |
| 8.0 → 7.5 | 17 fl oz |
| 8.2 → 7.5 | 24 fl oz |
| 8.4 → 7.5 | 31 fl oz |
| 8.0 → 7.2 | 26 fl oz |
Scale linearly with volume: a 20,000 gallon pool takes double. If your volume figure is a guess, fix that first — every number here rides on it.
Why alkalinity changes everything
Total alkalinity is the water's buffering capacity — its resistance to pH change. A pool with high TA needs far more acid to move pH, and a pool with low TA moves alarmingly on a small dose.
| Total alkalinity | Multiply the dose by |
|---|---|
| 60 ppm | 0.6 |
| 80 ppm | 0.8 |
| 100 ppm | 1.0 |
| 120 ppm | 1.2 |
| 150 ppm | 1.5 |
| 200 ppm | 2.0 |
A pool at 200 ppm TA needs twice the acid of the table above for the same pH movement — and its pH will climb straight back, because the buffer is still there. Chasing pH in a high-TA pool without addressing TA is a weekly ritual that never ends.
Lowering alkalinity uses the same acid, differently
There is no separate chemical. Acid lowers both, and which one moves more depends on how you add it.
To lower pH with minimal TA change: add acid slowly over a wide area with the pump running, well distributed.
To lower TA: add the acid in one spot in the deep end with the pump off, let it sit, then run the pump. This creates a low-pH column that strips carbonate out of solution. Then aerate — run a fountain, point a return jet up, turn on the spa jets — which drives off carbon dioxide and brings pH back up without bringing alkalinity back.
Repeat. That aerate-and-acid cycle is the only way to bring TA down, and it takes several rounds.
Dry acid
Sodium bisulfate, sold as pH Down. Easier to handle, no fumes, and it adds sulfates to the water — which never leave, and above about 300 ppm attack plaster and grout.
Dose is roughly 1.25 lb per 10,000 gallons for the 8.0 → 7.5 move above. On an indoor pool, or where handling liquid acid is genuinely impractical, it is reasonable. On an outdoor pool used all season, muriatic is the better long-term choice for the same reason liquid chlorine is: it leaves less behind.
Where the pH is coming from
Pools trend up, and knowing why tells you how often you will be doing this.
- Aeration — waterfalls, fountains, jets, and a salt cell's gas production all drive off CO₂ and raise pH.
- Salt systems — chlorine generation produces sodium hydroxide continuously. Salt pools need acid most weeks. This is the single largest hidden consumable in the salt comparison.
- New plaster — leaches calcium hydroxide for the first year. Expect heavy acid use and do not fight it too hard.
- Cal-hypo and liquid chlorine — both nudge pH up with every dose.
Trichlor tablets do the opposite, strongly. A tablet-fed pool usually needs alkalinity increaser, not acid.
Adding it safely
Acid into water, never water into acid.
Pump running. Pour slowly into the deep end, walking along the edge, away from the skimmer and away from any metal. Never pour into a skimmer — undiluted acid through the pump and heater is how heat exchangers fail.
Wait 20–30 minutes of circulation before retesting. A reading taken immediately reads the plume, not the pool.
Never mix with chlorine of any kind, in a bucket or anywhere else. Acid and hypochlorite together release chlorine gas.
Do it in the right order
pH is not the first adjustment. Total alkalinity is set first, because it governs how pH behaves; calcium and CYA come with their own sequence. Adjusting in the wrong order means doing several of them twice — the order of adjustment is the map.
And correct pH before shocking. At pH 8.2 only about a fifth of free chlorine is in its active form, so a shock dose delivered into high-pH water wastes most of itself — which is one of the reasons a pool can read high on chlorine and still be green.