Hot tub alkalinity: ideal range and how to raise or lower it

Total alkalinity in a hot tub should be 80–120 ppm. To raise it, add alkalinity increaser (sodium bicarbonate). To lower it, add pH decreaser (sodium bisulfate). Adjust alkalinity before you touch pH, because alkalinity is the buffer that keeps pH from swinging. Follow the product label for the dose for your tub's volume.

What total alkalinity is

Total alkalinity measures the dissolved carbonates and bicarbonates in the water. Think of it as a shock absorber for pH. With enough buffer, adding a chemical or a few bathers barely moves pH. With too little, pH jumps around. With too much, pH climbs and refuses to come down.

Alkalinity and pH are related but not the same thing. pH tells you where the water sits on the acid-base scale right now. Alkalinity tells you how hard it is to move.

Signs it is off

Low alkalinity (under 80 ppm)	High alkalinity (over 120 ppm)
pH bounces between tests	pH keeps drifting above 7.8
pH drops sharply after adding sanitizer or shock	pH decreaser seems to do nothing
Corrosion of heater and metal parts	Scale on heater, jets, and waterline
Stinging eyes, itchy skin	Cloudy water, weak sanitizer
Green tint from dissolved copper	Filter clogs quickly

How to raise alkalinity

1. Test the water and note alkalinity and pH.
2. Turn the jets on low.
3. Measure alkalinity increaser per the label for your tub's volume and how far you need to go. Raising in steps of 20–30 ppm is easier to control.
4. Add it to the water over the center of the tub.
5. Circulate for 20 to 30 minutes with the cover open.
6. Retest. Repeat if needed.
7. Then check pH. Raising alkalinity lifts pH a little. Adjust pH only after alkalinity is in range.

How to lower alkalinity

1. Test alkalinity and pH.
2. Turn the jets on low with air controls closed.
3. Add pH decreaser per the label. It lowers both alkalinity and pH.
4. Circulate 20 to 30 minutes and retest.
5. If alkalinity is now in range but pH is low, open the air controls, run the jets, and leave the cover off for 30 to 60 minutes. Aeration brings pH back up without raising alkalinity.
6. Repeat in small steps over a day or two if alkalinity started very high. Large single doses overshoot.

Why alkalinity changes

• Fill water. Some tap water arrives at 200 ppm or more. Well water varies widely. Test a fresh fill before adding anything.
• pH decreaser. Every dose lowers alkalinity.
• Bromine tablets. They are acidic and slowly pull alkalinity and pH down.
• Top-ups. Each top-up with hard, alkaline water adds a little more.
• Heavy use. Sweat and body oils are mildly acidic.

The pH drift problem

Hot tubs aerate water constantly, and aeration raises pH. The more alkalinity in the water, the faster and further pH rises. If your routine is "add pH down every three days," try this: let alkalinity settle around 80 ppm instead of 120 ppm. pH will rise more slowly and level off sooner. More on this in pH too high or too low.

Balance order after a fresh fill

1. Total alkalinity to 80–120 ppm.
2. pH to 7.2–7.8.
3. Calcium hardness to 150–250 ppm. See the calcium hardness guide.
4. Sanitizer to range.
5. Shock.

Let the water warm up to at least 80°F before judging the readings. Cold water dissolves chemicals slowly and can read differently.

Common mistakes

• Chasing pH while ignoring alkalinity. pH will not hold until the buffer is right.
• Adding pH increaser when alkalinity increaser was needed. Soda ash raises pH a lot and alkalinity a little. Sodium bicarbonate raises alkalinity a lot and pH a little. Use the one that matches the low reading.
• Adding two products back to back. Wait 20 to 30 minutes between products and never mix them.
• Testing right after shocking. High sanitizer can throw off strip pads. Wait a few hours.
• Dosing for the wrong volume. Check your tub's gallons in the manual. Most hold 250 to 500 gallons.

When it will not respond

If alkalinity and pH will not settle after several careful rounds, the water is likely past its useful life. Dissolved solids from months of dosing make water stubborn. Drain, refill, and balance from scratch using the ideal levels chart. It takes an afternoon and saves weeks of frustration.

If you would like a second opinion on your readings, photograph your strip and the AskTub strip reader will tell you which number to fix first.