How to Fix a Green Pool
Raise free chlorine (FC) to shock level for your CYA, brush and filter continuously, and re-test daily until the water clears.
Safety first
- Never mix chemicals together.
- Always add the chemical to the water — never water to the chemical.
- Wear gloves and eye protection when handling pool/spa chemicals.
- Keep chemicals out of reach of children and pets.
- Always follow the product label — it overrides any guidance here.
Why pools turn green
Algae is everywhere — in rain, on leaves, on swimsuits. Chlorine keeps it in check. The moment free chlorine (FC) drops near zero for even a day, algae can take hold and bloom within 24-48 hours, turning the water green, cloudy-green, or even black in shaded areas.
Step-by-step fix
- Test (or estimate) your CYA, then use the Pool Shock Calculator to find your shock-level FC target.
- Add chlorine to reach that target. Re-test every few hours — algae consumes FC fast at first.
- Brush the walls and floor to knock algae off surfaces and into the water where chlorine can reach it.
- Run the filter continuously (24/7) until the water is clear.
- Keep re-dosing FC back up to shock level each time it falls, until it holds overnight.
- Once clear, vacuum any dead algae off the floor and check your filter pressure — it will clog faster than usual.
- Re-test TA, pH, and CYA once stable, since heavy dosing can shift them.
How long does it take?
Light green, cloudy water can clear in 24-48 hours. Dark green or black water can take 3-7 days of daily dosing and continuous filtering. Be patient — adding more chlorine than the plan calls for won't speed this up and can waste product.
Frequently asked questions
Why did my pool turn green overnight?
Green water is almost always algae, triggered by low or zero free chlorine (FC) for a day or two — often after heavy rain, a power outage on the pump, or simply letting FC run out during hot weather.
Can I swim in a green pool?
No. Treat it as contaminated until it's clear and FC is back in the normal range.
Will shocking it once be enough?
Usually not for a fully green pool. Expect to re-test and re-dose FC daily for 2-4 days until it holds overnight, then filter until clear.
Do I need to vacuum or brush a green pool?
Yes — brushing walls and floor daily helps break up algae and keeps it in suspension so the filter can remove it.
How long does the filter need to run?
Run it continuously (24/7) until the water is clear, then back to a normal 8-12 hour schedule.