Hot tub pH: what it should be and how to move it

The water turns hazy, scale forms on the shell and the heater, and chlorine loses its punch. Hot Spring's manual puts the line at 7.8: above it, expect cloudy water and scale. Master Spas notes that above pH 7.5 less than half of the free chlorine is in its active form, so a high-pH tub can read enough chlorine and still be under-sanitized.

Acidic water attacks the tub. Low pH corrodes metal parts and the heater, can etch or stain acrylic, and burns through sanitizer faster. Pitted metal fittings are one of the signs Master Spas lists for low pH or alkalinity.

Check alkalinity first. Low alkalinity lets pH bounce, and while you adjust alkalinity the pH reading swings, so Hot Spring says to ignore it until alkalinity is set. Then use a spa pH increaser (sodium carbonate) to raise it or a spa pH decreaser (sodium bisulfate) to lower it, the two types Hot Spring names.

Hot Spring's list of don'ts includes pool muriatic acid for lowering pH. Its manual has chemicals go into the filter compartment with the jets on high for at least ten minutes, cover off. Master Spas instead says to spread them across the surface with the pumps running. Use your own maker's method.

Everything you add moves it a little, including sanitizer. Master Spas' manual says the chemicals used to sanitize and clean the tub can raise or lower pH, and that low alkalinity lets it fluctuate. If pH won't stay put, retest alkalinity, then see hot tub alkalinity.