There can be a lot of reasons why a pool turns green. Algae can form when conditions for growth are optimised. A high pH, a low chlorine level, lack of circulation and lack of filtration are just a few issues that people can find being the cause of algae growth. Ensuring that your filter is in good health, and running the correct amount of time each day can aid in the prevention of algae. Keeping an eye on pH and chlorine levels will also aid in prevention. An Algae preventative Algaecide is also recommended to ensure that algae growth is kept in control.