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Green Water - What Causes It and How to Get Rid of It

Green Water - What Causes It and How to Get Rid of It

So you've got your pool up and running, cleaned out all of the leaves and dirt, and everything's looking good, then one day you look out and notice that your pool is green or cloudy. What happened? In this two part entry we will talk about the different causes of green and cloudy water, how to troubleshoot to find the exact problem, and how to get the water clear again. In this first part we will tackle green water.

Causes and Effects of Green Water in Pools

There are three causes of green pool water. The first is a lack of chlorine. Without chlorine the pool water will quickly start to grow algae and turn green. 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 thing that causes pool water to look green (or yellow) is pollen. Pollen gets carried into the pool water by the wind and is too small for the sand filter to remove. As the pollen builds up in the water and settles on the liner, it gives the water a green or yellow look. Despite its unattractive look, pollen is completely harmless and has no ill effects on the yourself or the pool.

The last cause of green water in pool water is metals, usually copper, that have oxidized. Metals can be introduced through source water, from some cheap algaecides, or, if the water is acidic, from the metal components of the pool, most commonly from copper heater elements. Shocking the pool will oxidize these metals, and, if there are enough metals present in the water, turn the water green. If left untreated, high amounts of metals will stain the finish of the pool. Anyone with blonde hair that swims in a pool with excessive metals will also have their hair turn green.

Troubleshooting Green Water
A full algae bloom in a pool.

The first thing to do when troubleshooting the cause of green water is to look at the water itself. Each of those causes have subtle differences in how they make the water look. Algae, for instance, will start growing on the pool walls and bottom first. It is dark green, feels slimy to the touch, and will usually not come off unless 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 sit on the bottom of the pool, and is often mistaken as either green or yellow algae, however, unlike algae, pollen does not stick to the pool surface. If you wave your hand over the affected area (without touching it) and it puffs up like a cloud, then you're most likely dealing with pollen, not algae.
Pool water turned green by metals.
If your pool looks similar to this after
you shock the water, you may have a
metal problem.

Pool water turned green by metals will be a consistent light green colour, like the pool has had food colouring added to it. Despite the colour, the pool will still look fairly clear and you will still be able to see the bottom. It will present itself right after the pool has been shocked. If a clear pool is shocked and the turns green, you're most likely dealing with a metal issue.

To confirm your suspicions it is best to get your pool water tested at your local pool store, paying close attention to chlorine and copper. 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 the test shows that there is a chlorine residual in the water within that range algae is most likely not the problem. With that being said, algae can sometimes build-up a resistance to chlorine over time. For persistent algae blooms, the chlorine level should be kept at 5ppm or higher, until the water has cleared up.

Traces of metals in the water test results, along with a consistent, light green appearance to the water is most likely a metal issue. If the water has a chlorine residual and no sign of metals then pollen is the most likely cause.

Treating Green Water

Green water caused by algae should be treated by super-chlorination and algaecide. The algaecide should be a poly-quat with a strength of at least 40%, and preferably 60%. The entire surface of the pool should also be brushed when adding these chemicals to break up the protective coating that the algae cover itself with. As the algae is killed, the water will usually turn cloudy before going clear. During treatment, the 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 filter out the dead algae. 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.

To get rid of pollen in your pool water is trickier because sand filters don't filter fine enough to get catch the pollen. Clarifiers are also generally 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.
If your pool has metals, make sure to add
a sequestrant weekly to prevent staining
like this from happening.

To treat a metal issue add a sequestrant to the water. Sequestrants will 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 water is clear, it's best to then chemically clean the filter to remove any metals that have been trapped. You should then try to pinpoint how the metals got into the water. 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. To read more about pH and alkalinity, and how they affect your pool, click here. If metals are entering the pool from the source water, you should continue using sequestrants weekly to help filter them out.

As you can see, green water can mean more than just algae. Metals and pollen can also turn the pool green, and all three require very different treatments to remove. Knowing how to spot the difference between them, and getting your water tested, is the only way to know how to treat the issue, and get your pool clear. Check back next week for part two where we talk about how to properly treat cloudy water.

If you have any more questions about how to treat a green pool, or any other water care issue, call our certified water care specialists at 905-666-5333. Did you like this post? Like us on Facebook ( www.facebook.com/TheSpaShoppe) or follow us on Twitter ( @thespashoppe) to get notified when new blog entries are posted!