Pools - Green Swimming Pools Factsheet

Swimming Pools Green Swimming Pools Fact sheet for operators

What causes pool water to turn green?

Pool water turns green because of algal overgrowth. Green algae is a common problem in swimming pools where chlorine levels are too low. Chlorine levels can become too low because of a chlorine lock, photolysis or simply not enough chlorine.

Chlorine lock occurs when there is too much cyanuric acid put into the swimming pool (cyanuric acid is the stabiliser used to prevent the loss of chlorine by the sun). Chlorine lock occurs when cyanuric acid concentrations exceed 50ppm (the optimum concentration for cyanuric acid is 38ppm).

Photolysis is when UV light from the sun breaks down the chlorine molecules causing chlorine to evaporate, resulting in chlorine loss. Correct amounts of stabilisers will help prevent photolysis. Chlorine levels should be maintained between 1 ppm to 3 ppm.

How to change your pool from green to pristine

To rid your pool of algae try the following:

Vacuum any dead or brown algae from the swimming pool flooras this will help maintain your chlorine levels by reducing the nutrients in the water. Backwash the filter to remove any algae or nutrient material.

If you use a water stabiliser, check your stabiliser concentration. If it is more than 50 ppm, empty some water out of the swimming pool and refill it with fresh water until the stabiliser concentration is around 38 ppm. If you don’t have a test kit, your local pool shop can test the water for you.

Check your chlorine levels. If your chlorine levels are below 0.5 ppm, there is not enough chlorine to kill the algae.

Shock dose the swimming pool to 10 ppm, keep your filters running and do not use your pool until the chlorine level falls below 3 ppm.

Check your chlorine levels often to ensure at least 1 ppm concentration is maintained. If using a stabiliser again, adjust your chlorine concentration to at least 2.0 ppm.

What if the water is beyond saving?

If your swimming pool has not been maintained for a long period of time it will probably have a high level of algal growth and be a potential breeding ground for mosquitoes. Your swimming pool may require draining and cleaning.

This fact sheet was initially developed by Redland Shire Council, and is recognised and adopted by participating Queensland Councils.

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