A few things can cause green pool water. Firstly, it could be a lack of chlorine. Without sufficient chlorine your pool water will quickly start to grow algae and turn green. When algae is in bloom and the water is without chlorine, it is not safe to swim in, and can become a breeding ground for mosquitoes and other insects.
Another thing that causes pool water to appear green (or yellow) is pollen. The wind carries pollen into the pool water and it is too small for the sand filter to trap. As pollen builds up in the water and settles on the lining, it can give the water a green or yellow colour. It may not look good, but pollen is completely harmless and will have no ill effects on yourself or the pool.
Lastly, metals – usually copper – can oxidize and cause water to go green. Metals can be get into your pool via source water, through using some cheap algae killers, or – if the water is acidic – from some of the metal components in your pool (most likely from copper heating elements). Shock-treating the pool will oxidize these metals, and (if there are enough metals present in the water) cause it to turn green. If not treated, these high amounts of metals will ruin the finish of the pool. People with blonde hair that swim in a pool with too much metal will also see their hair turn green!
We hope that you found this article insightful.