How to Fix a Green Pool: A Step-by-Step Guide
Turn a green pool clear again: test, brush, shock to the right level for your stabilizer, run the filter and vacuum. Doses, timelines and mistakes to avoid.
How to Get Rid of Mustard Algae in a Pool
Yellow, dusty patches that brush away and come back? How to identify and kill mustard algae, clean everything it touches and stop it returning.
Key takeaways
- Mustard algae looks like yellow or brown dust on shaded walls and brushes off easily, then returns.
- Kill it by brushing and holding shock-level chlorine until the water passes an overnight chlorine loss test.
- Sanitize swimsuits, toys and pool tools, which can carry it back into the water.
On this page
Mustard algae, sometimes called yellow algae, is the stubborn cousin of green algae. It clings to shaded walls, looks like yellow sand or pollen and brushes away easily, only to return a few days later. Dark blue-black spots that barely brush off are a different problem: see our guide to black algae. The key is killing it everywhere it hides, not just where you can see it.
How to identify mustard algae
- Yellow, tan or brown patches, usually on shaded walls and steps
- Brushes off easily into a cloud, then settles back
- Returns in the same places within days
- Doesn’t turn the whole pool green like common algae
Pollen and dirt can look similar, but they don’t keep coming back to the same spots.
Step-by-step treatment
1. Test and balance
Test free chlorine, pH and stabilizer. Lower pH to about 7.2–7.4 so chlorine works at full strength; see lowering pool pH.
2. Brush everything
Brush all walls, steps, corners and behind ladders thoroughly. Mustard algae protects itself with a coating that chlorine struggles to penetrate.
3. Shock and hold
Raise free chlorine to about 40% of your stabilizer level and hold it there, testing and topping up several times a day. Our shock dosing guide shows how much to add.
4. Run and clean the filter
Run the pump around the clock and clean or backwash the filter often, as dead algae collects there. Our filter guide covers each type.
5. Sanitize everything else
Mustard algae can survive on swimsuits, toys, floats, brushes and vacuum heads, then reinfect the pool. Wash swimsuits, and soak toys and tools in a strong chlorine solution before rinsing.
6. Confirm it’s gone
Keep holding the shock level until the water is clear, combined chlorine is 0.5 ppm or lower and free chlorine drops by no more than 1 ppm overnight. The same test is explained in our green pool guide.
Why it keeps coming back
- Chlorine held too briefly or too low
- High stabilizer weakening chlorine; see our cyanuric acid guide
- Contaminated equipment or swimwear
- Poor circulation in shaded corners
Frequently asked questions
Is mustard algae dangerous?
It isn’t usually harmful to touch, but algae can harbour bacteria and hide swimmers in cloudy water. Treat it before swimming.
Do I need a special algaecide?
Some products target mustard algae, but a thorough brush-and-hold chlorine treatment is the essential part.
How long does it take to get rid of mustard algae?
Often several days of holding shock-level chlorine. Don’t stop early.
Sources
Every article is edited by a human and checked against our editorial policy. Spotted a mistake? Tell us.
Keep reading
Cloudy Pool Water: Causes and Fixes
Why pool water turns cloudy and how to clear it: low chlorine, high pH or alkalinity, poor filtration, high calcium, early algae and debris, with fixes.
How to Get Rid of Black Algae
How to identify and kill black algae: hard brushing, spot treatment, shocking to the right level, filter cleaning and how to stop the spots coming back.