Hot tub alkalinity: too high or too low
Keep hot tub alkalinity at 80–120 ppm: in 400 gallons, about 25 g (0.9 oz) of baking soda raises it by 10 ppm.
Always follow your product’s label directions. Pool and spa chlorine, bromine, shock and algaecides are EPA-registered pesticides: use products labeled for pool or spa use, and don’t swim or soak until levels are within the limit on your product’s label. If this page and a label disagree, the label wins.
On this page (7 sections)
In the same 400-gallon tub, about 38 g (1.3 oz) of 93.2% dry acid lowers alkalinity by 10 ppm.
Example: going from 60 to 90 ppm takes about 76 g (2.7 oz) of baking soda. Lower alkalinity in rounds, and run the jets between rounds to bring pH back up.
Target range and why TA drives pH
Total alkalinity (TA) is the amount of buffer in the water, mostly bicarbonate. It controls how hard it is to change pH. We aim for 80–120 ppm, Taylor's ideal range. The industry standard for spas accepts 60–180 ppm.
TA works in both directions:
- Too low (below about 60–80 ppm): pH swings with every dose and every soak, and tends to fall. Low pH corrodes the heater.
- Too high (above about 120 ppm): pH climbs steadily, because the jets drive carbon dioxide (CO2) out of the water. The more alkalinity there is, the more CO2 there is to lose. That's the most common reason a hot tub "always needs pH down".
If your tub's pH keeps rising, the lower half of the range (around 80 ppm, sometimes 60–80) is usually easier to manage. The full set of targets is in the spa water chart.
Alkalinity too high: acid + aeration method
Acid lowers both pH and TA. Aeration raises pH but doesn't change TA. Alternating the two lowers TA while bringing pH back each time:
- Add acid to bring pH down to about 7.2, no lower. With dry acid in a 400-gallon tub, lowering TA by 10 ppm takes about 38 g (1.3 oz). Keep each round to about 20 ppm of TA or less.
- Aerate. Take the cover off, run the jets on high, and open the air controls. pH will rise over several hours to a day. Alkalinity stays where the acid left it.
- Retest pH and TA when pH is back near 7.6. Repeat until TA reaches your target.
At 20 ppm per round, bringing TA from 160 down to 100 takes three rounds, usually over a few days. Don't soak while pH is below 7.2. The same method is explained for pools in lowering pool alkalinity; the numbers here are for tub-sized volumes.
Alkalinity too low: baking soda dose
Baking soda (sodium bicarbonate, also sold as "alkalinity increaser") raises TA with only a small effect on pH. Add no more than about 30 ppm at a time. Sprinkle it across the water with the jets running, and retest after 30 minutes. If pH is also low after TA is right, finish with soda ash. See raising low pH.
| Tub size | Raise: baking soda | Lower: dry acid 93.2% | Lower: muriatic acid 31.45% |
|---|---|---|---|
| 200 gal | 13 g (0.45 oz) | 19 g (0.65 oz) | 15 mL (3 tsp) |
| 250 gal | 16 g (0.55 oz) | 24 g (0.85 oz) | 18 mL (3.8 tsp) |
| 300 gal | 19 g (0.65 oz) | 29 g (1.0 oz) | 22 mL (4.6 tsp) |
| 350 gal | 22 g (0.8 oz) | 34 g (1.2 oz) | 26 mL (5.3 tsp) |
| 400 gal | 25 g (0.9 oz) | 38 g (1.3 oz) | 30 mL (1.0 fl oz) |
| 450 gal | 29 g (1.0 oz) | 43 g (1.5 oz) | 34 mL (1.1 fl oz) |
| 500 gal | 32 g (1.1 oz) | 48 g (1.7 oz) | 37 mL (1.2 fl oz) |
| 600 gal | 38 g (1.3 oz) | 58 g (2.0 oz) | 45 mL (1.5 fl oz) |
| 700 gal | 44 g (1.6 oz) | 68 g (2.4 oz) | 53 mL (1.7 fl oz) |
Acid amounts are rounded down. Multiply by the number of 10 ppm steps you need, and remember the acid will lower pH too.
Calculator
Enter your gallons, current TA, and target. If the target is higher, you get a baking soda dose. If it is lower, you get an acid dose for the whole change. Split an acid dose into rounds of about 20 ppm with aeration in between, as described above. Always follow the product label.
Why it won't move
- TA won't go down: aeration alone doesn't lower TA; only acid does. Acid without aeration drops pH and stops you from adding more. You need both, one after the other.
- It comes back after a refill: your tap water may be high in alkalinity. Test the fill water once so you know your starting point.
- Too much pH up: soda ash adds alkalinity as well as raising pH. Repeated pH-up doses push TA up over time.
- TA won't go up or keeps falling: bromine tablets and frequent MPS (non-chlorine) shock are mildly acidic and slowly use up alkalinity.
- The test is the problem: many strips read TA in steps of about 40 ppm, so a "change" of 10–20 ppm may not show. A drop test (titration) reads in 10 ppm steps.
High pH with high TA is covered on why hot tub pH keeps rising.
Safety
- Never mix acid with chlorine or bromine products. Wait at least 30 minutes between them with the jets running.
- Add chemicals to water, never water to chemicals. Wear gloves and eye protection when handling acid.
- Stay out of the tub while pH is below 7.2 during the lowering rounds.
- If acid is swallowed or splashed in the 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
How do I raise the alkalinity in a hot tub?
Add baking soda: about 25 g (0.9 oz) per 10 ppm in a 400-gallon tub. Sprinkle it with the jets running, add no more than about 30 ppm at once, and retest after 30 minutes.
What to do if alkalinity is high in a hot tub?
Lower it in rounds: add acid until pH is about 7.2, aerate with the cover off until pH is back near 7.6, retest, and repeat. Each round lowers TA; aeration restores pH without adding alkalinity back.