In this guide: Why pools go green · The SLAM method · Green, mustard & black · Preventing a comeback · Calculators
A green pool means algae has outrun your chlorine. The fix isn't a single dose — it's holding a high, steady chlorine level long enough to kill every cell, while brushing and filtering out the debris. Done properly, most green pools clear in a few days.
Algae blooms when free chlorine falls too low (a broken feeder, a heatwave, a big party, or a storm dumping organics and phosphates). Warm, still, sunlit water accelerates it. Low or absent stabilizer lets the sun strip chlorine faster than it can work.
1. Test and balance pH to about 7.2 — chlorine is far more effective at the low end of the range.
2. Brush everything, especially walls, steps, and shady corners, to break the algae's grip and expose it to chlorine.
3. Shock to the algae-killing level for your stabilizer and hold it there — this is a high free-chlorine target, dosed to your exact gallons. A weak dose just wastes chemicals.
4. Run the filter 24/7 and clean it as pressure rises; dead algae clouds the water before it clears.
5. Maintain the high level, re-testing and re-dosing several times a day, until free chlorine holds overnight and the water is clear. That "holds overnight" test is how you know the algae is dead.
For the full walk-through with the chemistry explained, see our green pool rescue guide.
Green is the common, fast-blooming type that clears with SLAM. Mustard (yellow) clings to shady walls, brushes off easily, but returns — it needs a sustained higher chlorine hold and thorough equipment cleaning. Black algae roots into plaster with a protective cap; it must be scrubbed open with a stiff brush before chlorine can reach the roots, and is the most stubborn of the three.
Hold free chlorine in range every day, keep stabilizer around 30–50 ppm outdoors, run enough turnovers, brush weekly, and shock after heavy bather loads or storms. Occupancy matters too — a crowded pool burns chlorine fast, so know your bather load.
These calculators turn the steps above into your exact numbers:
This guide is built on published health codes, national standards, and government energy guidance. For the exact equations behind the linked calculators, see our formula reference.
This guide is general information, not a substitute for your local health code or a licensed professional. Always follow the dosing instructions on your chemical labels and confirm readings with a test kit.