Dealing with Green Pool Water: Causes, Treatments, and Best Practices

Nothing is more frustrating than looking for a green pool that was crystal clear the day before! Although this may be frustrating, it is one of the unlucky perks of owning a pool. So, when the pool is green, here is what to do!

When a pool is green, it is a sign of excessive algae growth in the water. This can easily be rectified with the use of chlorine. A pool shock treatment with a minimum level of 70% chlorine will kill the algae and return the water to a crystal clear color within 3 to 4 days.

Read on to discover what causes pool water to turn green, how to treat it, and how to prevent it from occurring again.

What Turns A Pools Water Green?

The greenish color of the pool water comes from live algae in the water. This can happen when there is an imbalance in the pool water. Algae are always present in the water, and it is the job of the chemicals and pool water filter to keep them in check.

So if the pool water is not being circulated through the filter sufficiently or frequently enough, or there is an imbalance of chemicals, this may lead to excessive algae growth. If the pH level of the pool water is too high, it will prevent chlorine from doing what it does best, kill algae.

It is also common for a pool to turn green after a heavy rain storm. This is due to the raindrops that carry dust and pollen from the air and drop it into the pool water. The pollen, dust, and metals found in the raindrops will react to the chemicals in the pool water, raising the pH level and promoting algae growth.

So, if your pool has become a shade of green, there is no need to worry! It can be rectified quickly and easily. The earlier it is diagnosed, the easier it will be to bring the pool back to a brilliant blue.

What To Do When Your Pool Is Green?

The only efficient way to bring your green pool water back to a crystal clear is by using chlorine, a lot of chlorine! This is accomplished by using a pool shock treatment. The shock treatment contains high levels of chlorine and other chemicals that have been specially blended to remove algae from the water. So, follow these easy steps to bring your pool water back from a green infestation.

Clean The Pool Walls

It is vital to clean the walls, steps, and floor of the pool, as this is where the algae cling to. This can be done using a medium soft bristle brush for fiberglass pools. For concrete pools, it is recommended to use a heavy-duty bristle brush. The algae can be tenacious, so some vigorous scrubbing may be required to release it from the walls.

The scrubbing will release the algae from the pool surfaces and suspend them in the pool water. This will allow the chlorine to do its magic and kill it.

Test The Pool Water pH Level

To ensure the shock treatment works effectively, it is essential to have the pool water’s pH and alkalinity levels in the correct range. Use a liquid pH test kit or pH test strips and adjust the levels as needed.

The optimum pH level should be between 7.4 and 7.6. The optimum alkalinity levels are between 100 and 160 parts per million (ppm).

If the levels are slightly lower than optimal, it will suffice, as the pool will be receiving shock treatment with chlorine.

Add Pool Shock To The Water

The pool shock will kill the algae and sanitize the pool water, so using a pool shock with a minimum of 70% chlorine is essential. The best time to add the shock is in the early evening when the algae are at their most vulnerable. The amount of shock that must be used will depend on the algae type and the pool size.

A standard shock treatment for a pool without any algae growth is 1lb per 10,000 gallons of water. For pools that have algae growth, the dose may need to be increased. This is important to be able to treat the water and get rid of the algae.

Algae Color


Shock Required

Light blue green

Minimal algae growth

Double shock amount (2x)

Dark green

Substantial algae growth

Triple shock amount (3x)

Black green

Invasive algae growth

Quadruple shock amount (4x)

Run The Pool Filter

After adding the shock treatment to the pool water, let it mix for about 30 minutes before running the pool filter. Let the water run through the pool filter overnight or for at least 8 hours. For quicker results, let the pool filter run continuously till all the algae are gone.

After letting the filter run overnight (8 hours), it is recommended to back-wash the filter to clear out the algae. If the pool water still has a light green hue, it will need to be shocked the following night again. Repeat these steps till the water has a cloudy light blue hue. The cloudy-blue color means all the algae have been successfully removed.

Continue running the pool filter for another 24 hours to ensure all the dead algae have been filtered from the water.

Final Water Testing

Now that all the algae have been killed and filtered out of the water, you should be left with a blue or cloudy-blue water color. Test the pool water’s pH, calcium, and chlorine levels. Add pool chemicals as needed to bring the water back within its parameters. Adding a pool water clarifier will make this process much easier but be sure to test the water after 24 hours of adding it.

How To Prevent A Pool From Turning Green?

The best way to keep your pool sparkling blue is regular pool maintenance. This can be done by testing the water weekly and keeping the pH and alkalinity level constant. Use a pool cover if you live in a region with high dust and pollen levels.

Using an algaecide is an effective way to prevent algae growth. The downside of this is that not all algaecides are made the same. Some are better than others, so stay safe and regularly test the pool water.

Keep the pool filter clean by back-washing regularly and replace the sand with a proper filter sand every 3 years.


When your pool water has a green color, it is a sign that there is an excessive algae build-up. This can easily be treated with chlorine in the form of a pool shock. The amount of pool shock that must be used will depend on the water’s color. The darker green the water is, the more algae growth there is. Regular pool maintenance will prevent excessive algae growth.