What Causes pH Changes in a Pool?

What Causes pH Changes in a Pool

If you have a pool, then you probably already know that maintaining the pH is important. You use the little kit and test the water, and sometimes, you’ll find that the pH is higher than it should be. Why does that happen? What causes high pH in a pool? There are five common causes, and when you understand them, it can make pool maintenance a little bit easier.

Carbon Dioxide

Carbon dioxide lowers the pH of water. In fact, it can lower your pool’s pH without changing the alkalinity, which is why carbon dioxide aeration is a nice technique for controlling pH.

The process involved here is that carbon dioxide combines with water to create carbonic acid. As the name suggests, this acid does in fact lower water’s pH.

This means that if your pool loses carbon dioxide, the pH will go up. Large amounts of splashing (like after a pool party), water features, and anything else that agitates the water can release carbon dioxide dissolved in the water. That will ultimately lead to a higher pool pH.


Since carbon dioxide can lower the pH, and algae consume carbon dioxide in order to live, it makes sense that algae can raise the pH of your pool.

Algae can consume a lot of CO2 very quickly, and that’s really the whole story here. Kill the algae, and it will be easier to keep the pH down to a safe and desirable level. If algae gets out of control, then you’re going to see that pH creep upward. A pH at the low end of the acceptable level, between 7.2-7.6, combined with certain sanitizers, actually helps prevent algae growth.


This is where a lot of people assume pH control takes place, and that’s not a wrong assumption. We all put a lot of things in our pools to keep them clean and sanitary, and every additive has its own impact on the pH of the pool. As an obvious example, acids lower the pH, and they change the alkalinity.

Conversely, several types of chlorine can raise the pH of a pool. Calcium hypochlorite, sodium hypochlorite, and salt chlorine generators are all known for this behavior. As they release chlorine into the pool, the process also raises the pH.

But not all types of chlorine raise the pH. Some liquid chlorines have a net-zero impact on pH. Because of that, it’s always best to check the pH after you treat the pool. You might need to adjust the ph, or you might not need to do much.


Etching is one of the most deceptive and easy-to-miss reasons for an increase in pH. This really only applies to concrete pools, but they’re common enough that it’s worth covering.

If you add too much acid to your pool too quickly, you can actually raise the pH in the pool. It won’t happen immediately. If you check the pH a few minutes after adding acid, the numbers will be low. But that pH can bounce back up pretty quickly, and it can be confusing. That’s because adding too much acid at once decreases total alkalinity, which is a buffer for large ph swings in the pool.

The issue here has to do with the LSI. If you aren’t familiar, LSI is a way of measuring the chemical balance in a pool, and it’s closely related to calcium levels in the water.

When you add a lot of acid to your pool very quickly, it changes the LSI, and the water will naturally seek out more calcium in order to stabilize. If the water is sitting inside of a concrete barrier, it has access to all of the calcium it could need. The water will etch into the concrete to increase the calcium concentration in the water. That process actually raises the pH of your pool, and it can completely counteract the impacts of the acid you put in there.


You might have noticed that a lot of these causes of high pH are completely natural. People like to use their pools, and that leads to splashing and removing carbon dioxide from the water. Algae are always trying to grow in the water, and etching happens naturally too.

For all of these reasons, it’s common for the pH in your pool to go up over time. It’s why pool care and maintenance need to be consistent. If you don’t check your pH regularly, you will most likely see it get higher with time.

If you want to simplify pool maintenance, or if you need any type of pool work, Select Pool Services is here to help. We’ll gladly share our expertise with you, so never hesitate to come to us with your pool questions. And when it’s time for work, we’d be delighted if you allowed us to take care of your pool. Contact us today!

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