Green Pool Guide

My Pool Has Gone Green - Why? How Do I Fix It?

Green pool water is unsightly, and it can be startling to find this condition developing in your pool. If your pool’s water chemistry is generally well-balanced, you may be wondering: why is my pool green?

In this article, we will cover the most common causes of green pool water and then provide a step-by-step guide for how you can restore your pool’s water to healthy, clean conditions. We also provide a couple of tips for preventing green pool water from returning in the future.

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Causes Of Green Pool Water

Green pool water is almost always caused by algae growth. If you are startled by unexpected green pool water, you may be unknowingly neglecting certain aspects of your water chemistry that can allow algae to flourish.

In this section, we’ll review some aspects of water chemistry that can allow for algae growth while also discussing one other factor that may be causing your green pool water.

Low Chlorine Levels

When low chlorine levels are present, algae and bacteria can spread rapidly. Because chlorine is the main sanitising component in your pool water, free chlorine levels should be maintained at 2-4 ppm (parts per million) to maintain clean and healthy water conditions.

High Cyanuric Acid

High cyanuric acid can also lead to green pool water by causing chlorine inefficiency. Ideal CYA levels in your pool range from 30-50 ppm. At these levels, cyanuric acid protects your pool’s chlorine from degrading in the sunlight.

Above this range, there is simply too much CYA bound with the chlorine molecules in your pool, making them unable to neutralise active contaminants like algae and bacteria.

High Phosphates

High phosphates in your pool can lead to more severe algae infestations. However, because phosphates only act as a food source for algae, it is unlikely for high phosphates to be the _only _cause of your green pool.

Phosphates are a naturally occurring compound that typically enters pools in small amounts through natural debris and swimmers. Because phosphates are not inherently bad in a swimming pool, it is often debated what levels of phosphates necessitate phosphate removal.

The consensus is that at levels reading above 500 ppb (parts per billion), you should use a phosphate removal product in your pool.

If you are experiencing a green pool, it may be wise to treat any phosphate levels that rise above 300 ppb. Please note that removing phosphates simply depletes the food source of the algae and will not kill or remove the algae itself.

High pH Levels

Similar to high CYA levels, high pH levels are correlated with chlorine inefficiency. Ideal pH levels range from 7.2-7.6. At high pH levels, the oxidation-reduction potential of your pool’s free chlorine molecules is drastically reduced, effectively eliminating their ability to neutralise contaminants.

You can learn more about your pool’s oxidation-reduction potential in our article here.

Metals In Your Pool Water

In cases where green pool water is not caused by algae, it is most often due to oxidising metals found in your pool’s water. Copper is the most likely culprit for a green pool as it typically turns green as it oxidises, but sometimes iron can also be the source of this unsightly hue.

To confirm the presence of metals in your pool’s water, we recommend taking a water sample to your local pool supply store.

Many of these businesses offer metal testing for pool water at little or no cost to you. If these services are not available, you may want to invest in copper testing strips.

Green Pool Treatment

Use A Shock Treatment

When treating a green pool caused by algae, you should start with a shock treatment. Both chlorine and non-chlorine-based shock treatments are effective for this purpose.

Shock your pool following the instructions provided on your shock treatment of choice. If you do not notice significant improvements in the quality of your pool’s water, consider performing an additional shock treatment 12-24 hours after the initial dose.

After using a chlorine-based shock treatment, ensure that your pool’s chlorine levels return to pre-shock levels before balancing the rest of your pool’s chemistry. This can take up to 48 hours.

Use this pool shock calculator to help you work out the right dose of shock for your pool.

Remove Phosphates

Before removing phosphates, we recommend testing their levels in your pool using a test kit. When experiencing a green pool, you should remove phosphates once their levels read at 300 ppb or higher.

To remove phosphates, use a pool chemical known as a starver. These are available in regular strength, but can also be found in more concentrated formulas for those experiencing more than 1,000 ppb of phosphates in their pools.

We recommend Lo-Chlor’s range of phosphate starvers, beginning with Lo-Chlor Starver in its original intensity.

Balance Pool Chemistry

After shocking your pool and removing phosphates if necessary, return your pool’s chemistry levels to normal. To begin, ensure that your pool’s free chlorine levels range from 2-4 ppm.

Add more chlorine if necessary, keeping in mind that levels above 4 ppm are not actually more effective and can cause discomfort for swimmers.

You should also test your pool’s CYA levels using a test kit. If your CYA levels are reading above 50 ppm, you will need to lower the cyanuric acid in your pool.

This can be done by partially draining and refilling your pool’s water, or by the use of bio-active pool products.

It is also a good idea to test your pH levels to ensure they fall within the acceptable range of 7.2-7.6. To lower pH levels in your pool, you will need to use an acid. Typically, you must use either hydrochloric acid or sodium bisulphate, which is a safer-to-handle dry acid.

Add An Algaecide

Algaecides are designed as an algae-prevention method. Therefore, these products should not be used as a first line of defence in the algae removal process.

However, once your pool is reasonably free from most algae and green-coloured water, adding an algaecide can be a great way to kill off any remaining traces of algae while protecting your pool for months to come.

If you experience issues related to dissolved metals in your pool, we recommend avoiding copper algaecides.

Although copper-based algaecides are highly effective, they can cause issues in pools that already have large amounts of metals present.

Remove Dissolved Metals

Before making efforts to remove dissolved metals from your pool water, ensure that they are present in the first place. Testing pool water for metals can be done at most local pool supply stores, but you can also find metal testing kits available online.

To remove metals from your pool, you will need a metal removing product. These products bind with the dissolved metals in your pool, preventing them from oxidising and turning green while also keeping them from settling on the surfaces of your pool in the form of stains.

Preventing Green Pool Water

If you find high phosphates in your pool, this can allow algae to return more quickly. Pools with an abundance of natural debris tend to develop high phosphates.

If you live in an area where your pool regularly accumulates plant matter, you may want to add phosphate testing and removal to your monthly pool maintenance regime.

Another step you can take to prevent green pool water from occurring in the future is maintaining your pool with an algaecide. One dose of an algaecide product can prevent algae from developing for up to three months!


Green pool water can be unsightly enough to scare off even the keenest of swimmers. Luckily, green algae in pools has straightforward remedies. After ridding your pool of green water, your pool will be clean and safe for your family to enjoy once more.

Do you have any questions about green pool water or how to treat it? Get in touch with us in the comments section, we’d love to help answer any remaining queries you may have!

Louis from Pool Advisor


A chemical engineer by trade, Louis is committed to debunking myths in the pool industry by explaining the underlying chemistry and making it accessible to all.