Green pool water is almost always caused by algae — and algae grows when your chlorine level drops too low. The good news is it's fixable.
Why it happens
Chlorine levels dropped below 1 ppm. Heavy rain diluted your chemicals. High heat and sunlight burned off your chlorine faster than normal. Pump or filter wasn't running enough hours per day.
Step 1: Test your water
Check pH, chlorine, and alkalinity. You need to know where things stand before adding chemicals.
Step 2: Balance pH first
Your pH needs to be between 7.4 and 7.6 before shocking. Outside this range, shock is much less effective.
Step 3: Shock the pool
For a green pool, use 2 to 3 times the normal shock dose. Add it directly to the water in the evening — sunlight breaks down shock quickly.
Step 4: Run the pump continuously
Keep your pump running 24 hours a day until the water clears. This circulates the chemicals and pushes water through the filter.
Step 5: Brush the walls and floor
Algae clings to surfaces. Brush everything thoroughly to break it loose so the chemicals can kill it.
Step 6: Vacuum to waste
Once the algae is dead it turns gray or white. Vacuum it up using the Waste setting on your filter so it doesn't clog your filter.
Step 7: Clean or backwash your filter
Your filter will be working overtime — clean it thoroughly after clearing up a green pool. A lightly green pool clears in 24 to 48 hours. A heavily green pool may take 3 to 5 days.
Prevent it from happening again
Test your water twice a week. Shock every week during swim season. Run your pump at least 8 hours per day. Keep a supply of chlorine on hand so you never run out.