What causes pool water to turn green?

Pool water turns green because of algal growth. 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 stop the loss of chlorine by the sun). Chlorine lock occurs when cyanuric acid concentrations are above 50 mg/l (the ideal concentration for cyanuric acid is 38 mg/l).

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

How to change your pool from green to pristine

To get rid of algae try the following:

• vacuum any dead or brown algae from the swimming pool floor as this will help maintain your chlorine levels. Backwash the filter to remove any algae. 
• if you use a water stabiliser, check your stabiliser concentration. If it is more than 50 mg/l, empty some water out of the swimming pool and refill it with fresh water until the stabiliser concentration is around 38 mg/l. 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 mg/l, there is not enough chlorine to kill the algae
• shock dose the swimming pool to 10 mg/l, keep your filters running and do not use your pool until the chlorine level falls below 3 mg/l
• check your chlorine levels often to ensure at least 1 mg/l concentration is maintained. If using a stabiliser, adjust your chlorine concentration to at least 2 mg/l.

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 breeding mosquitoes. You might have to empty and clean your swimming pool.