Hot tub pH too low: how to raise it
In a 400-gallon hot tub with alkalinity at 80 ppm, about 17 g (0.6 oz) of soda ash raises pH from 7.1 to 7.4.
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On this page (7 sections)
Soda ash is sodium carbonate, sold as pH increaser or pH up. About 0.7 oz in total gets you to 7.5 in two steps. If total alkalinity (TA) is below 80 ppm, raise it with baking soda first, or pH will drop again.
What low pH does to the tub and to you
Low pH makes water corrosive. In a hot tub, the damage shows up in expensive parts:
- Heater and metal parts. Acidic, hot water attacks the heater element and metal fittings. The damage builds up slowly, so a tub can run at low pH for months before a part fails.
- Shell and cover. Low pH combined with low calcium can etch surfaces and age the cover's underside faster.
- Skin and eyes. Stinging eyes and itchy skin are common complaints when pH drops below about 7.2.
- Sanitizer swings. Chlorine is very active at low pH but is used up faster, so readings drop quickly between soaks.
The CDC's acceptable range is 7.0–7.8; test kit makers use 7.2–7.8. We aim for 7.4–7.6. Most kits can't read below 6.8, so a reading at the bottom of the scale may be even lower.
Common causes (bromine tabs, acid overdose, low TA)
- Bromine tablets. They are mildly acidic and dissolve all the time, so tubs on a bromine floater tend to drift down in pH and alkalinity over weeks. See bromine tub care.
- Too much pH decreaser. A pool-sized scoop of dry acid in a few hundred gallons will overshoot. In a 400-gallon tub, lowering pH from 7.8 to 7.5 takes only about 9 g of dry acid at TA 80.
- Low total alkalinity. Below about 60 ppm, there is little buffer, so even small additions push pH around. This is the most common reason pH "won't stay put."
- Frequent MPS shock. Non-chlorine shock (MPS) is mildly acidic and slowly lowers pH and TA.
- Soft or acidic fill water, and rain in an uncovered inflatable tub.
How much pH increaser: it depends on alkalinity
The calculator at the top raises pH by no more than 0.3 per step and rounds down, so you can retest before adding more. Soda ash raises pH strongly and also adds alkalinity. Always follow the product label.
| TA | pH 7.2 → 7.5 | pH 7.0 → 7.3 |
|---|---|---|
| 40 ppm | 7.0 g (0.2 oz); TA +4 ppm | 10 g (0.35 oz); TA +6 ppm |
| 60 ppm | 10 g (0.35 oz); TA +7 ppm | 15 g (0.5 oz); TA +10 ppm |
| 80 ppm | 14 g (0.45 oz); TA +9 ppm | 20 g (0.7 oz); TA +13 ppm |
| 120 ppm | 21 g (0.7 oz); TA +13 ppm | 31 g (1.0 oz); TA +19 ppm |
The same pH rise takes about three times more soda ash at TA 120 than at TA 40. Higher alkalinity resists change in both directions, which is why it keeps pH steady once it is in range. Sprinkle the dose across the water in front of a jet with the jets running, leave the cover open, and retest after 30 minutes. Soda ash can cloud the water briefly; it should clear as it mixes.
Alkalinity or pH first?
Alkalinity first, when it is out of range. Taylor's instructions say to bring alkalinity into range before adjusting pH, and this order works in practice:
- TA below 80 ppm and pH low: add baking soda to bring TA to 80–100 ppm. Baking soda raises pH only a little, but by raising TA it stops pH from sliding back. Then retest pH and top up with soda ash if still needed. In a 400-gallon tub, 25 g (0.9 oz) of baking soda raises TA by 10 ppm.
- TA in range and pH low: use soda ash (pH increaser) directly.
- TA high and pH low: unusual. Retest with fresh reagents first. If it is real, raise pH by aeration (jets and air on, cover off) rather than adding more alkalinity.
When pH won't go up
- Check TA. At 40 ppm or less, pH rises and falls back quickly. Fix TA first.
- Check the test. Old strips and reagents give wrong readings. Very high chlorine (above about 10 ppm, for example right after a shock) also distorts pH readings (PHTA): wait until chlorine drops, then retest.
- Look for a constant acid source: a bromine floater set wide open, or MPS added daily.
- Give it time. Wait 30 minutes with the jets running before judging a dose.
- Refill if the numbers make no sense. If several doses do nothing and your tests disagree with each other, fresh water is often faster than chasing the numbers.
Rising pH is the more common hot tub problem: see hot tub pH too high. The hot tub chemical calculator checks every range from one test.
Safety
- Add one product at a time. Never mix pH increaser with acid or chlorine in a container.
- Add chemicals to water, never water to chemicals. Soda ash dust irritates eyes: pour low and wear eye protection.
- If someone swallows pH increaser or gets it in their eyes, call Poison Control at 1-800-222-1222.
- Always follow the product label. It is a violation of federal law to use a pesticide product in a manner inconsistent with its labeling.
FAQ
Why is my hot tub pH always low?
Most often because alkalinity is too low to hold pH, or because a bromine floater keeps adding acidic tablets. Frequent MPS shock and soft fill water also contribute. Raise TA to 80–100 ppm with baking soda first, then adjust pH.
Does hot tub shock raise pH?
It depends on the product. Non-chlorine shock (MPS) lowers pH slightly. Dichlor is close to neutral. Liquid chlorine and calcium hypochlorite (cal-hypo) raise it a little. None of them is a reliable way to correct pH.
Should I raise alkalinity or pH first in a hot tub?
Alkalinity first if it is below 80 ppm. Once TA is in range, raise pH with soda ash in small steps and retest after 30 minutes.
Sources
- CDC, Home pool and hot tub water treatment and testing (pH 7.0–7.8): cdc.gov
- Taylor Technologies, K-1005 instruction manual (alkalinity before pH, pH test limits): PDF
- CDC, Model Aquatic Health Code 2024, Annex (pH and chlorine): cdc.gov (PDF)
- PHTA / AQUA Tech Notes, MPS non-chlorine shock: PDF