pH Level of Your Hot Tub Water: Why You Almost Always Need pH-

In this article, we explain exactly what the pH value is, why tap water often doesn't have the correct starting pH level for a hot tub, and why you almost always need pH- during use, and less often pH+.

What is the pH value, and why is it important?

The pH value indicates how acidic or alkaline water is, on a scale from 0 to 14. A value of 7 is neutral. Anything below that is acidic; anything above that is alkaline. Alkaline means that a substance has the exact opposite effect of an acid: it neutralizes acids and often feels smooth or soapy, as with baking soda or soapy water.

For a hot tub, the ideal pH range is approximately 7.2 to 7.6. Within that range, your disinfectant works optimally, the water stays clear, and you prevent irritation to the skin, eyes, and mucous membranes. Outside that range, problems arise: if the pH is too low, the water will corrode rubber, plastic, and metal parts. If the pH is too high, your disinfectant will be less effective, and you’ll experience limescale buildup and cloudy water more quickly.

Tap water often has a pH that is already too high by default

The European standard for drinking water specifies a pH between 6.5 and 9.0. In practice, each country maintains the water at a pH between approximately 7.4 and 8.2. This is a deliberate choice, as this range is better for the pipes through which the water flows.

That's too high for your hot tub. If you fill the tub with fresh tap water, you'll almost always start with a pH level that's just above the ideal range. That means you'll need to add pH- when filling your hot tub to bring the level to the correct value.

Why the pH rises again during use

Even if you've adjusted the pH properly, it won't stay stable. As soon as you turn on the jets or bubbles, you'll see the pH level rise again after a while. There's a logical explanation for this.

Water contains carbonic acid, which is formed from dissolved CO2. This carbonic acid makes the water slightly more acidic. Movement, heat, and the bubbling of air cause CO2 to escape from the water. Warm water retains gases less effectively than cold water anyway, and agitating the water while using jets or bubbles accelerates that process even further. When the carbonic acid disappears, so does the acidic effect, and the pH rises.

This is the main reason why pH- is the product you use most often in practice. You really only need pH+ if you accidentally add too much pH- and the pH level becomes too low.

How often should you check the pH level?

How often you need to test the water depends on how often you use the hot tub.

• For daily use: check the pH at least every two to three days.
• After a session with jets or bubbles: check the pH the next day, because the level usually rises by then.
• If you have any doubts about water quality, if the water is cloudy, or if it has an unusual odor: test it immediately.
• After topping off with fresh water: always retest, even if the pH was correct before.

How do you test the pH level of your hot tub?

The most practical way to measure pH is with water test strips. You hold the strip in the water for a few seconds, compare the color to the corresponding color chart, and can immediately see whether you need to add pH- or pH+. Test strips provide a clear result within a minute and are accurate enough for normal home use.

In a nutshell

• Tap water typically has a pH that is slightly too high for a hot tub.
• The use of jets and bubbles causes CO2 to escape, causing the pH to rise again during use.
• As a result, you generally need pH- more often than pH+.
• Check the pH level with test strips at least every few days, and always after heavy use or when adding water.

Maintaining a stable pH level isn't complicated—it's simply a matter of measuring it regularly and making adjustments in a timely manner. With the right products on hand, you can keep your water clear, your hot tub in good condition, and your skin feeling great after every session.