What Makes a Pool Turn Green?
Green pool water is almost always algae. Algae are microscopic organisms that blow in on the wind, hitch rides on swimsuits, or sneak in with rain. They're always present in the environment — the question is whether your pool chemistry is giving them a reason to party.
When conditions are right — warm water, low chlorine, unbalanced pH — algae multiply fast. A pool can go from clear to green in as little as 24–48 hours during a hot stretch of weather.
3 Root Causes of Green Pool Water
1 Your Chlorine Dropped Too Low
This is the #1 cause. Chlorine is the only thing keeping algae at bay. When free chlorine drops below 1 ppm — especially in warm water above 80°F — algae can establish themselves quickly. This often happens after a big pool party, a heat wave, or a few days of skipped testing.
2 Your pH or Alkalinity Is Way Off
Even if you have chlorine in the water, if your pH is too high (above 7.8), that chlorine becomes largely inactive. You'll see 3 ppm of chlorine on your test strip — but it's not actually killing anything. Algae love this situation. If alkalinity is off too, the pH problem compounds.
3 Your CYA (Stabilizer) Is Too High
CYA above 80 ppm causes chlorine lock — your chlorine is essentially neutralized. Your test says you have plenty of chlorine, but it can't do its job. The result? Green water even though the numbers look okay. The only fix for high CYA is partial draining and refilling.
How to Clear a Green Pool: Step-by-Step
Test your water first
Grab a test kit or test strips and check pH, alkalinity, chlorine, and CYA. You need a baseline before adding anything. If you skip this, you'll likely overdose one thing and underdose another.
Balance pH and alkalinity
Get pH to 7.2–7.4 (slightly on the low side of normal) and alkalinity to 80–120 ppm. Chlorine works best in this range. If pH is high, lower it with muriatic acid or dry acid before shocking.
Brush the pool walls and floor
Before adding chemicals, physically brush every surface — walls, steps, corners, behind the ladder. This breaks up algae colonies and exposes them to the chemicals you're about to add. Don't skip this step.
Shock the pool — hard
Light green pool: 2 lbs of cal-hypo per 10,000 gallons. Dark green or black pool: triple-shock — 3 lbs per 10,000 gallons. Add shock at dusk or night so UV doesn't burn it off before it works. Pour slowly around the perimeter.
Run the filter continuously
Don't turn the pump off. Run it 24 hours a day until the water clears. Your filter is what physically removes dead algae from the water — not the chemicals alone.
Backwash or clean your filter often
Dead algae clog the filter fast. Backwash every 12–24 hours while clearing the pool. If you have a cartridge filter, rinse the cartridge each time. A clogged filter slows everything down.
Add an algaecide (optional but helpful)
Once the chlorine level drops back to normal range — after about 24 hours — adding a maintenance algaecide helps prevent a comeback, especially for persistent mustard algae.
Re-test and adjust
After 24–48 hours, test again. Water should be turning from green → cloudy blue → clear. Adjust chlorine and pH as needed and keep running the filter until the water is fully clear.
🧪 Not sure how much shock to add? PoolDiag calculates the exact dose based on your pool size and current readings.
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