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Why Do Pools Go Green?

So you’ve got your pool up and running and everything’s looking good, then one day you look out and notice that your pool water is green. What happened? Today we will look at what causes pool water to turn green, how to troubleshoot to find the reason why the pool went green, and how to get the water clear again.

What Causes Pool Water To Turn Green?

There are three reasons why pool water goes green.

A Lack Of Chlorine

The first, and most common, reason why pool water goes green is a lack of chlorine. Without chlorine in the water the pool will quickly start to grow algae. If chlorine is not added to the water once this process starts, the algae can grow to a point where the entire pool turns green within a day. The warmer the water is, the faster this process will be.

Note: Water without chlorine, especially if it has an algae bloom, is unsafe to swim in, and can be a breeding ground for mosquitoes and other insects.


The second reason why pool water might look green (or yellow) is pollen. Pollen is produced by nearby plants and trees and is carried into the pool water by the wind. Since pollen is too small for a standard pool filter to remove, the amount of pollen will continue to build throughout the year.

As the amount of pollen builds up in the water it begins to settle on the liner, and can be mistaken for either green or mustard algae. If the pollen level continues to build it can eventually turn all of the water in the pool green. Fortunately, pollen is completely harmless and has no ill effects on the yourself or the pool.


The last cause of green pool water is metals, usually copper. Metals can be introduced through many sources; such as source water, cheap algaecides, or if the water is acidic, from the metal components of the pool such as copper heater elements.

When you shock a pool with metals in the water, the metals will oxidize. If there are enough metals present in the water, this process can turn the water green. If left untreated, the problem will continue to get worse and will stain the finish of the pool, potentially permanently. Anyone with blonde hair that swims in a pool with excessive metals will also have their hair turn green.

Troubleshooting Green Pool Water

So now that you know the possible causes of green pool water, the question becomes how to tell which one is affecting your pool. To do that requires some troubleshooting.

Step 1 – Look At The Pool

The first thing to do when troubleshooting the cause of green pool water is to look at the pool itself as each potential cause of green water has subtle differences in how they make the water look. Algae will start to grow on the pool walls and bottom first. It is usually dark green will feel slimy to the touch. Algae will also stick to the pool walls and won’t come off unless the walls and floor of the pool are scrubbed. As the algae bloom worsens the water will begin to look swampy; consistently dark green and impossible to see more than a few inches into the water.

Like algae, pollen will first become visible on bottom of the pool. For this reason it is often mistaken for green or yellow algae. Unlike algae however, pollen does not stick to the pool surface. If you wave your hand a couple of inches over the affected area (without touching it), pollen will puff up like a cloud where algae will not move. Since pollen doesn’t stick to the surface of the pool, you won’t often see it on the sides of the pool, where algae is commonly found stuck to the pool walls and bottom.

Pool water turned green by metals will be a consistent light green colour; looking almost as though the pool has had green food colouring added to it. Despite the colour, the water will still look fairly clear and you should be able to see the bottom of the pool.

Since metals in the water are most affected by pool shock, if a clear pool turns green right after being shocked, you’re most likely dealing with a metal issue. Metals can also stains to appear on the surface of the pool. If you have a green spot on your liner that won’t scrub off, you likely have a problem with metals in the pool water.

Step 2 – Test The Pool Water

Once you’ve had a good look at the water, it is best to then get your pool water tested at your local pool store. Be sure to mention to the person testing your water that you’re dealing with green water, and describe to them what the water looks like. Bringing along some photos of the problem can also be very helpful.

When going over the results of your water test, pay close attention to chlorine and copper levels in the water. If there is no chlorine present in the water, and the water is a dark green, then the most likely cause is algae.

Since algae usually cannot survive in water that has a chlorine level of 1-3ppm (parts per million), if your test shows that there is a chlorine residual in the water of at least 1ppm, algae is most likely not the problem.

If your water test results show traces of metals in the water, and the water has a consistent, light green appearance, you’re most likely dealing with a metal issue. If the water has a good chlorine residual and there is no sign of metals, pollen is the most likely cause of your green pool water.

How To Clear A Green Pool

The treatment for green pool water depends on why the pool water went green in the first place.

Getting Rid Of Algae In Pool Water

Algae should be treated by adding chlorine shock and algaecide to the pool water. Why not just shock? Algae protect themselves with a slimy coating that can greatly reduce the effects of chlorine. While most algaecides don’t actually kill algae on their own, they do break up this protective layer, allowing the chlorine to kill the algae much more easily.

Along with adding chemicals, you should also brush the entire surface of the pool. This helps to break up the algae blooms into smaller pieces that can be more easily killed by the chemicals.

As the algae is killed, the pool water will usually go from green to cloudy before eventually going clear. This process can take a number of days. During this time, the pool pump should be left running 24 hours a day to allow for an even distribution of chlorine to the water, and allow the filter to remove the dead algae that is causing the cloudiness. Until the water is completely clear you should also check your chlorine level at least once a day, adding additional chlorine every time the level dips below 1ppm.

Getting Rid Of Pollen In Pool Water

To get rid of pollen in your pool water is trickier because most pool filters don’t filter fine enough to actually catch and remove the pollen. Most clarifiers are also relatively ineffective on pollen. Skimmer socks placed over the skimmer basket will help to remove any pollen that is suspended in the water, but need to be frequently changed, and will never completely clear up the issue. In our experience, adding a flocculant is the most effective treatment for removing pollen. For more information on how to treat a pollen problem, talk to one of our water care specialists.

Getting Rid Of Metals In Pool Water

To treat a metal issue in your pool requires the use of a sequestrant. Sequestrants essentially group the metal particles together to allow the pool’s filter to more easily filter them out.

After the sequestrants have done their job and the pool is clear again, you now need to remove the trapped metals from your filter. While a simple backwash will remove most of them, it’s best to chemically clean the filter to ensure that all of the trapped metals are removed from the pool. If you’ve never chemically cleaned your filter, talk to one of our water care specialists for more information.

After all of the metals have been removed from the pool, you should then try to pinpoint how the metals got into the water in the first place.

If your water test showed a low alkalinity/pH, and you have a heater, the cause was likely rust from the heat exchanger. Bypass the heater, if possible, and immediately raise the alkalinity/pH. If metals are entering the pool from the source water, you should continue using sequestrants weekly to help filter them out on a regular basis.

Wrapping Up

As you can see, there are many causes for green pool water. Metals, algae and pollen can all turn your pool green. Knowing how to spot the difference between them, and getting your water tested, is the only way to properly treat the issue and get your pool clear.

If you have any more questions about how to treat a green pool, or any other water care issue, call one of our certified water care specialists.


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