Five Common Causes for Green Pool water

If there’s something that sends a chill down a pool owner’s spine, then it’s the sight of having green pool water when everything seems to be going along fine. Today we’re going to take a look at the five most common reasons why pool water turns green.

Low Chlorine Levels

This is the most common cause for green pool water and this is why here at Mr Pool Man, we always stress the importance of testing your pool water and keeping your free chlorine levels (FC) at around 1ppm to 3ppm. When your free chlorine levels drop below 1ppm or to zero, you basically don’t have anything that combats algae growth and the result? Green pool water! So don't forget to test, test, and test your water on a regular basis! For more information on how to raise low chlorine levels in pool water, click here.

High Cyanuric Acid Levels

If you’re using chlorine pucks or stabilised pool chlorine, there’s a good chance that you have excess cyanuric acid levels in your pool. Keeping your cyanuric acid levels at the recommended levels is a balancing act (pun intended) on its own. Too low a cyanuric acid level will lead to your chlorine being burned away by the UV rays of the sun and too high of a cyanuric acid level will render your chlorine ineffective, causing green pool water even if you have chlorine in your pool.

For more information on lowering your cyanuric acid levels and how to deal with chlorine locks, you can check out this article here.

High pH levels

Another reason for getting green pool water even if you have adequate pool chlorine levels is having a high pH level. High pH levels, much like high cyanuric acid levels, can render your pool chlorine ineffective. What’s too high pH levels? You can get away with a pH level of 8.0 but the higher it goes from there, the more ineffective your pool chlorine will be. This is fairly important to note especially if you’re using a salt water chlorinator since salt water chlorination typically leads to higher pH levels than traditional chlorination with liquid or granulated chlorine.

High phosphate levels

Phosphates are not something that is commonly tested (or included) in many test kits. Even if you have the right free chlorine levels, having a high phosphate level can still lead to green pool water. Why? Phosphates are what we consider an algae superfood, so even if you have the proper pool chlorine levels, you can still end up with green pool water! Phosphates usually come from organic pool debris, rain, and runoff from your lawn.

Metals in your pool water

This final cause for green pool water has nothing to do with algae, but it is quite alarming when it happens. Green pool water that’s caused by metals usually happens once you refill your pool with new water and you shock it. When your pool water has high levels of copper or iron (common in well water) once you apply chlorine or other pool chemicals to it, you will see that it will turn green in absolutely no time.

Many people will mistake this for an algae bloom and will add more chlorine to the pool, which really doesn’t do anything and they’ll start to wonder what went wrong. The biggest thing that you have to remember is that if you’re adding NEW water to the pool and the pool water turns green, the likely culprit is metals and not algae. To treat this, you will need to add a sequestrant to your pool. This doesn’t actually take out metals from your pool water, but it justs prevents it from coming out of solution and tinting your pool water green. Another option to remove metal from your water is to get some chelating agents that extract the metal particles in your pool water and traps them.

SEE ALSO: Help! My Pool turned Green after Shocking!


Knowing what causes green pool water can go a long way to preventing it, and we all know that prevention is much better than curing it, especially when it comes to pool algae and green pool water. By knowing how to prevent green pool water, you're saving yourself the time and the hassle of shocking your pool multiple times and hours of scrubbing. But if you halready have a full-blown aglae infestation on your hands then it's time to refer to our guide on how to deal with green pool water here.

Do you have any questions about this topic or the featured products? No worries, we're here to help! Drop us a question down below and we'll get back to you ASAP.

Happy swimming :)

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