Green water means algae, and algae means your chlorine has lost the fight. The good news is most cases reverse in 48 to 72 hours. The bad news is the chemistry is more particular than YouTube tutorials suggest.
Honest steps to try yourself
- Shock the pool with chlorine. The dose depends on volume and current chlorine level, follow the product instructions exactly, not "more is better".
- Run the filter continuously for 24 to 48 hours. Algae dies, then needs to be filtered out, that's often the slowest step.
- Brush the walls and floor. Algae clings to surfaces, brushing puts it into suspension so the filter can grab it.
- Check pH, ideally 7.2 to 7.6. Chlorine is dramatically less effective outside this range.
- Backwash or clean the filter once the water clears. Otherwise you're recirculating dead algae.
Be honest: by the time the pool is green, the chemistry has been off for at least a week. The shock fixes the symptom. Fixing the cause takes a proper service.