4 Reasons Your Pool Stays Green Despite High Chlorine

Too much cyanuric acid (CYA) CYA binds to chlorine and makes it less effective. Once CYA exceeds 50–60 ppm, you need much higher free chlorine to kill algae, even if test strips show high levels.

Phosphates from fertilizers Phosphates feed algae and can overwhelm chlorine. They are a common secondary cause when CYA is under control.

Mustard algae resistance Mustard algae is chlorine-resistant and requires specific treatment. Standard shocking often fails against it.

Low pH Chlorine works best between pH 7.2 and 7.6. Low pH reduces its killing power, letting algae survive.