Hot tub pH too high or too low: how to fix it

Hot tub pH should be 7.2–7.8. If it is too high, add pH decreaser (sodium bisulfate). If it is too low, add pH increaser (soda ash). In both cases check total alkalinity first and get it to 80–120 ppm, because alkalinity is what holds pH in place. Dose per the product label for your tub's volume and retest after 20 to 30 minutes.

What high and low pH do

	pH too low (under 7.2)	pH too high (over 7.8)
Comfort	Stinging eyes, dry itchy skin	Itchy skin, red eyes
Water	Clear but aggressive	Cloudy, dull
Equipment	Corrodes heater elements, pump seals, and metal fittings	Scale on heater, jets, and shell
Sanitizer	Chlorine is strong but used up quickly	Chlorine is weak. At pH 8.0 only about a fifth is active.
Other signs	Green tint from dissolved copper, etched surfaces	Rough white deposits, clogged filter

How to lower high pH

1. Test pH and total alkalinity. Note both numbers.
2. Turn the jets on, air controls off. You want mixing without extra aeration.
3. Measure pH decreaser per the label for your tub's volume. If in doubt, use half the dose.
4. Add it to the water over the center of the tub, or pre-dissolve it if the label says so.
5. Circulate for 20 to 30 minutes with the cover open.
6. Retest. Repeat with a smaller dose if needed. Sneak up on the target instead of overshooting.

pH decreaser lowers alkalinity too. If your alkalinity was already near 80 ppm, lowering pH may push it too low. In that case, raise alkalinity back up and let pH settle.

How to raise low pH

1. Test pH and total alkalinity.
2. If alkalinity is under 80 ppm, raise that first with alkalinity increaser (sodium bicarbonate). It will lift pH along with it, often enough that you need nothing else. See the alkalinity guide.
3. If alkalinity is fine and pH is still low, add pH increaser per the label.
4. Circulate for 20 to 30 minutes and retest.

A free way to nudge pH up without adding anything: run the jets with the air controls open and the cover off for 30 to 60 minutes. Aeration raises pH without raising alkalinity.

Why pH will not stay put

It keeps rising

This is normal for hot tubs. Jets, blowers, and waterfalls all aerate the water, which drives off carbon dioxide and pushes pH up. The higher your alkalinity, the faster it climbs. If you add pH decreaser every few days, let alkalinity sit at the low end, around 80 ppm, and the drift slows.

It keeps falling

Look for an acidic input. Bromine tablets are acidic. So are trichlor pool tablets, which should not be used in a hot tub at all. Low alkalinity is the other cause: with no buffer, each dose of anything moves pH a lot.

It bounces up and down

Alkalinity is under 80 ppm. Raise it and the bouncing stops.

It will not come down

Alkalinity is over 120 ppm and is buffering against you. Keep adding pH decreaser in measured doses over a day or two to bring alkalinity down, then set pH.

Testing tips

• Do not trust a pH reading taken within a few hours of shocking. High sanitizer distorts the pH pad on a strip.
• Test from elbow depth, away from where you just added product.
• Test when the water is at normal temperature, not during a cold refill.
• If strips are hard to read, see how to read test strips.

Safety

• pH decreaser is an acid. Keep it off skin and away from eyes, and do not breathe the dust.
• Never mix it with chlorine or any other chemical. Acid plus chlorine releases toxic gas.
• Add one product at a time and wait 20 to 30 minutes before adding another.
• Do not use muriatic acid in a hot tub unless a product is specifically labeled for spas. The margin for error in 300 to 500 gallons is small.

When pH problems mean something else

If pH and alkalinity will not hold in range no matter what you add, the water is probably old. After 3 to 4 months of dosing, the dissolved solids are high and the water stops responding predictably. A fresh fill is cheaper than fighting it. Check the full set of targets in the ideal levels chart and start clean.

Repeated low pH also shortens heater life. If you have had acidic water for a while and the tub starts tripping its GFCI, a corroded heater element is a likely cause.