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What Causes High pH in a Pool? (8 Likely Reasons)

High pH in a pool is bad news. It’s harsh on your skin and eyes, it clouds up your water, it scales your pool surfaces, and it also weakens your sanitizer.

Unfortunately, pH is way more likely to drift upwards than downwards in a swimming pool, so a mysteriously rising pH is something you’re going to face as a pool owner.

This article will look at what might be causing high pH in a pool, how to prevent it from happening, and how to fix it if your pH has already skyrocketed.

8 Likely Causes for High pH in a Pool

There are dozens of possible reasons for rising pH in your pool, but many of them are unlikely at best.

Here are the most common reasons:

1. You Recently Shocked Your Pool

Shocking your pool is a necessary part of your regular maintenance schedule. It “resets” your water’s sanitizing power by oxidizing used up chlorine (combined chlorine) and taking out any contaminants your normal sanitizer level is unable to deal with.

Depending on what type of shock treatment you’re using, however, this process may be responsible for your high pH reading.

The most popular type of pool shock treatment is calcium hypochlorite, commonly shortened to “cal hypo”. This is granular chlorine with a high pH level of 12.

As you might imagine, adding this alkaline substance to your water will raise the pH level of your water along with an increase in calcium hardness, albeit a small increase.

If you happen to be using other types of shock treatment, such as dichlor, trichlor, or chlorine-free shock (MPS), you don’t need to worry about your pH rising as these are either pH neutral or acidic substances.

Liquid chlorine does have a high pH level but the small rise in pH caused by this type of chlorine is short-lived, effectively making it pH neutral.

Prevention: If you’re using calcium hypochlorite to shock your pool, pair it with a suitable dose of muriatic acid to bring the pH back down and effectively nullify the increase in pH.

2. You Unknowingly Used a pH Increaser

You’ve seen products marketed at increasing the pH of your pool when it needs a boost, right?

They go by various names, including:

  • pH increaser
  • pH up
  • pH plus
  • pH booster

Few pool owners realize these products are really just baking soda (sodium bicarbonate), soda ash (sodium carbonate), or borax (sodium borate) with fancy branding.

While you’re unlikely to accidentally use soda ash in your pool, the same can’t be said for baking soda and borax.

For example, baking soda can be used to raise total alkalinity, clean pool surfaces, or sometimes even spot-treat algae. Borax is also sometimes used to clean pool stains or sticky residue.

If you didn’t know that baking soda and borax are also pH increasers, you probably won’t be aware of the impact they have on your pH.

Prevention: Avoid using household products on your pool if you’re not sure of their side effects, particularly if you already have trouble maintaining your pH level.

3. You Topped Off Your Pool with High pH Water

All pools lose water through things like evaporation, leaks, and splash out.

When that happens, it’s important to top off your pool in order to maintain an ideal water level. With that being said, the water you use to fill your swimming pool can make a noticeable difference to your pH (in either direction), even if you’re just topping it off.

High alkalinity water can come from any source, be it city water or well water, so you can’t always assume a pH range based on where you got it from.

Another factor that’s often overlooked is the total alkalinity of the water source. The lower the total alkalinity, the less of a buffer the pH has, and the easier it is to correct when it’s too high.

In other words, while high pH fill water can be a problem, it’s even more of a problem if the total alkalinity is high as well.

Prevention: Use a pool test kit to get a pH reading of an untested water source before adding it to your pool. If it’s highly alkaline, you may need to pair it with muriatic acid to counter the pH increase, depending on how much water is used.

4. Your Pool Has Algae

Pools with algae are problematic for a number of reasons.

One of them, surprisingly, is how it affects the pH level of your pool water. Algae consume sunlight along with minerals like nitrates and phosphates, all of which it can source from the comfort of the water.

But that’s not all.

It also feeds on the carbon dioxide (CO2) in the water. Since carbon dioxide is an acidic chemical compound, the water pH will gradually move in the opposite direction as more of it gets consumed.

Left untreated, this effect compounds as the algae continue to grow, leaving you with a very high pH level.

While there are ways to inject carbon dioxide back into your water, this will rarely be necessary if the conditions of your water prevent algae from forming in the first place.

Prevention: Stay on top of algae blooms by maintaining a suitable sanitizer level and regularly shocking your pool.

5. You Have a Salt Water Pool

Salt pools tend to have a higher pH compared to traditional chlorine pools. This is partly because they don’t use acidic chlorine tablets, but also because of the way saltwater generators work.

Saltwater generators turn salt into chlorine, but there’s an unintended side effect that has a small but measurable effect on pH, and it’s something few saltwater pool owners are even aware of.

During the conversion process, salt generators create small bubbles that aerate the water, which releases carbon dioxide from the water at a slightly higher rate than usual. This effect is also known as “off-gassing”.

Again, since carbon dioxide is acidic, losing more of it from the water will naturally cause the pH of the water to become more basic.

Of course, if your pH is rising while your salt generator is turned off, you can safely rule this one out as the root cause of your rising pH problem.

Prevention: The best way to counter this off-gassing effect is to lower your total alkalinity to 70-80 ppm. Lowering alkalinity will create a stronger opposing pull on your pH level, effectively keeping it more stable.

6. You Have Water Features

If you’ve installed water features on your pool such as waterfalls, statues, or misters, you’ll need to be more vigilant with your pH level.

Why does it matter?

Well, water features create a lot of movement on the surface of the water, which increases its surface area, which then causes the water to lose more carbon dioxide than normal. (Yes, it’s the same effect caused by saltwater generators.)

If you remember, this process is called aeration and it’s often used to deliberately raise the pH level of pool water.

Even having your return jets pointed too far upwards can aerate the water enough to see a noticeable increase in pH levels.

Of course, if you see this increase in pH while your water features are turned off and your jets are configured correctly, you can safely rule this one out as a possible cause.

Prevention: Don’t run your water features longer than necessary, and if your pH is already at the ideal upper limit (7.8 max), be prepared to lower your pH prior to turning them on to account for the additional increase.

7. You Recently Plastered Your Pool

If you have a plaster or aggregate pool, there’s a good chance it’ll need to be replastered at some point.

Resurfacing a plaster pool creates a problem, however.

As your new plaster cures, it releases calcium hydroxide into the water, which has a very high pH level of 12. As such, the increase in your water’s pH level depends on how much calcium hydroxide your water absorbs.

If your water is starved for calcium (meaning it has a low LSI), it’s going to seek more calcium hydroxide from the plaster, and you will see a significant spike in the pH of your water as a result. (The actual chemistry is a bit more complicated but that’s the gist of it.)

When you consider that new plaster can take 30 days to cure, it’s easy to see how this etching can have a dramatic effect on your water over time, not to mention compromise the plaster.

Prevention: Balance your LSI to prevent (or slow) the water from absorbing calcium hydroxide from the curing plaster. An LSI between 0 and 0.3 is optimal, anything less is going to make your water aggressive.

8. Your Total Alkalinity is High

You already know high pH pushes total alkalinity up, but did you know total alkalinity is a factor in rising pH in itself?

This goes back to dissolved carbon dioxide.

When alkalinity is high, it causes carbon dioxide to outgas faster than normal. Since this is an acidic substance, when you remove it from the equation it naturally increases the pH of your water.

It’s a lot like what happens when you leave a bottle of soda open for too long… it eventually goes flat.

What Goes Up, Must Come Down.

It’s not always obvious what’s causing high pH in your pool, but it’s really just a process of elimination.

The good news is, once you’ve identified the cause, a measured dose of muriatic acid, dry acid, or sulfuric acid should get you back on track, assuming your total alkalinity isn’t off the charts.

Finally, if rising pH is a recurring issue in your pool, consider installing an acid feeder to automate the process.

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