Why is my pool green?
Almost always, it’s algae, and algae means the sanitizer couldn’t keep up. Here’s why that happens and what to do about it.
The short answer
Algae spores get into every pool from wind, rain and swimmers. Chlorine kills them before they can grow. When chlorine drops too low, even for a few days, algae takes over and the water turns green. In hot weather that can happen fast.
The usual causes
Low chlorine. Heat, sun, heavy use and rain all use it up. The pump isn’t running enough. Water that doesn’t circulate doesn’t get filtered or sanitized. A clogged or failing filter. Dirt and algae stay in the water. See filter cleaning.
pH out of range. High pH makes chlorine work much less effectively.
Too much stabilizer. Cyanuric acid protects chlorine from the sun, but too much slows chlorine down.
Debris and nutrients. Leaves, pollen and storm runoff feed algae and eat chlorine.
A salt system that stopped making chlorine. See salt systems.
What kind of green is it?
Light green or cloudy green: early algae growth. Easier to clear.
Dark green, can’t see the bottom: a full bloom. Takes more work and filter cleaning.
Yellow or mustard-colored film on walls: often mustard algae, which is stubborn and comes back if not fully treated.
Dark spots that won’t brush off: may be black algae, which roots into the surface.
Clear green water: sometimes it’s metals in the water rather than algae, especially right after shocking.
What to do first
1. Keep everyone out of the water until it’s clear and balanced.
2. Make sure the pump is running and the baskets are empty.
3. Check the filter pressure. A green pool loads a filter fast.
4. Test the water so you know where chlorine and pH actually are.
5. Brush the walls and floor.
Safety: never mix pool chemicals together, and add chemicals to water rather than water to chemicals. Follow the label.
When to call
If you can’t see the bottom, the equipment isn’t working, or you’ve been fighting it for more than a few days, it’s usually faster and cheaper to have it handled. We’ll look at the pool, or a photo to start, and give you a plan and a price before we begin. More on our green pool cleanup service, or see a real green pool project.
Keeping it from happening again
Consistent chlorine, balanced pH, enough pump run time and a clean filter. That’s what weekly service takes care of.
Pool already green?
Text us a photo and roughly how long it’s been. We’ll tell you what it’ll take.
Green pool questions
How fast can a pool turn green?
In hot weather with low chlorine, algae can take hold within a few days.
Can I swim in a slightly green pool?
It’s best not to. Algae means the sanitizer isn’t keeping up, and cloudy water makes it hard to see the bottom.
Will shock alone fix it?
Sometimes, for a mild case. But if the cause is a pump, filter or chemistry problem, the algae usually comes back until that’s fixed.
How much does a green pool cleanup cost?
Green pool cleanups start at $500 plus applicable sales tax, depending on the pool’s size and how dirty it is. Most take about a week to clear.