Your pool was blue last week and now it’s green. So you’re asking the question every pool owner asks at some point: why is my pool green? The answer is pool algae. Tiny algae spores ride in on wind, rain, swimsuits, and pool toys, and when your water gives them a chance, they bloom fast.
Here’s the good news: you can fix it yourself, and most green pools are swim-ready again in about two days. Ignore it, though, and one bloom can turn into repeat outbreaks all season. In this post you’ll find out which type of algae you have, exactly how to kill it, and the simple weekly habits that keep it gone.
What Kind of Pool Algae Do I Have?
Check the color. That tells you what you’re fighting and how hard the fight will be. In your pool, you’ll almost always see one of these three:
- Green: The most common type. It turns the water cloudy green or grows slimy patches on walls and floors. It’s also the easiest to kill.
- Yellow (mustard): Looks like sand or pollen stuck to shady walls. It brushes off easily but comes right back if you don’t treat it.
- Black: Small dark spots, usually smaller than a dime, with roots that dig into your pool’s surface. It’s the rarest and the hardest to kill.
Fun fact: pink “algae” isn’t algae at all. It’s a bacteria that just looks like it.
How Do I Get Rid of Pool Algae?
Grab a pool brush, a vacuum, chlorine shock, and an algaecide. Keep everyone out of the pool until treatment is done. Can you swim with algae in the pool? It’s not usually a health hazard, but cloudy water makes it hard to see swimmers and judge depth, so wait until it’s clear.
How to Kill Green Algae
- Brush the walls, floor, and steps with a nylon bristle brush.
- Shock your pool with extra chlorine and run the pump for a full 24 hours.
- Add algaecide (follow the label) and run the pump for another 24 hours.
- Vacuum up the dead algae and clean your filter.
Kermit said it’s not easy being green. Lucky for you, green algae is the easy one.
How to Kill Yellow (Mustard) Algae
- Brush every affected wall first.
- Shock with a higher dose of chlorine than you’d use for green algae. Bad outbreaks may need two to four rounds.
- Vacuum and clean your filter.
Don’t skip the brushing or the shocking. If you do, yellow algae will be back before you know it.
How to Kill Black Algae
Black algae is stubborn because it has a tough outer shell and roots that grip your pool’s surface.
- Follow the green and yellow algae steps above.
- Scrub every spot with a stainless steel algae brush to break the shells and pull out the roots. Go carefully so you don’t scratch your pool finish.
- Keep scrubbing and shocking until every spot is gone.
How Do I Keep Pool Algae From Coming Back?
You’ve cleared your pool. Now let’s keep it that way. Killing algae takes a weekend, but preventing it takes a few minutes a week. Stop pool algae before it starts with these three habits.
Keep Your Water Moving
Algae loves still water. Run your pool pump every day so all your water gets filtered. Pay extra attention to “dead zones” like corners, the deep end, and behind ladders, where water barely moves.
Want backup? In-floor cleaning systems and specialty jet nozzle arrays push water along your walls and floor so algae has nowhere to settle. Building a new pool or upgrading the one you have? Request a quote and we’ll help you pick the right setup for your backyard.
Keep Your Chemistry Balanced
- Keep pH between 7.2 and 7.6. Above 8.0, chlorine stops working well and algae moves in. Test your pool water every week and adjust with a pH increaser or decreaser.
- Keep chlorine steady. Not a fan of the chlorine smell? Ozone and UV systems can cut your chlorine needs by up to 90 percent.
- Use algaecide as prevention. It works better at stopping pool algae before it starts than at killing a full bloom.
Keep It Clean
- Brush your walls, floor, and steps every week, even when they look clean.
- Vacuum after brushing so loose algae doesn’t settle back down.
- Let an automatic or robotic cleaner grab leaves, bugs, and grass before they become algae food.
- Shock your pool after big rainstorms or pool parties (we see you, 20 kids).
- Check sneaky spots like steps and ladders for early growth.
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