Why is My Pool Green?

In Blog, Chemistry, Maintenance Tips by Ed Miller

Algae. It’s the most common cause of water turning green. It’s important to understand how the algae got there in the first place so it can be prevented in the future, and then we’ll discuss treatment.

Algae is most commonly caused by of a lack of sanitizer in the water. If the pool water chemistry has not been maintained, LSI is off, and there’s no chlorine in the water, this becomes a breeding ground for algae. Consistent water quality is key for pool water in all aspects of aquatics. If your pool is green, it’s probably because the water quality is poor. Correct this and maintain it to prevent green water, and many other issues.
Poor water filtration or circulation can also contribute to algae growth. Ensure that water is being properly filtered and that circulation is where it needs to be. Algae will thrive at “dead spots” in the pool. Brush the bottom and sides of the pool weekly to prevent any growth there, paying attention to any dead spots such as corners, stairs, etc.


Start by balancing the pool to LSI. (See: How to use LSI for my pool?) If the pool was just filled, this may be all that is needed. After that, shock the pool. That may be enough if the algae is caught soon enough. If a properly balanced pool, that’s been shocked, continues to show signs of algae, you’ll need an algaecide. Avoid copper algaecides as they can stain the surface of the pool if not used correctly.
Brush the walls and floors of the pool to remove any algae present there. Vacuum pool well as the algaecide will also cause cloudy water. Use a clarifier and a filter-aid to bring back clarity. Using algaecide will increase load on filters, so expect increased backwash schedule. Depending on what algaecide is used, there could be foaming on the water that will also need to be addressed.
In some cases of stubborn algae, treat with a phosphate remover. After algae is gone, use a maintenance dose of algaecide to prevent it from reappearing. It’s common for algae to hide, and once a pool has an algae bloom, it’s easier for it to reappear.
Other causes for a green pool

In some rare cases, the green pool could be copper. Check all metal content in the water and treat with a metal remover if needed.
If CYA is astronomically high, it can work with algae, so if using CYA check that it’s not too high.