Why Is My Pool Green? Causes and a 4-Step Fix

A pool turns green for one main reason: algae grew because the free chlorine dropped too low for your stabilizer (CYA) level. Hot weather, heavy rain, a missed weekend or a pump that ran short all speed it up. The good news is that the fix is the same every time.

What turns a pool green

Algae. Algae spores arrive with wind, rain and swimmers every day. Chlorine kills them as they land. When chlorine runs low, they multiply faster than it can keep up, and the water goes from hazy to light green to dark green within days.

Chlorine that was too low for the stabilizer. Stabilizer (cyanuric acid, or CYA) protects chlorine from sunlight but also slows it down. With 30 ppm of CYA, 2 ppm of chlorine is enough. With 80 ppm, the same 2 ppm lets algae grow. A common rule is to keep free chlorine at about 7.5% of your CYA level or more: 3 ppm with 40 ppm of CYA, 6 ppm with 80.

Metals. If the water is clear but tinted green, or it turns green right after you shock, the cause is often copper. It comes from copper algaecides, ionizers, a corroding heater or some well water.

Step 1: Test before you buy anything

Test free chlorine, pH and stabilizer. A pool store can do it if you don’t have a kit. Two numbers decide everything else:

• Stabilizer (CYA) sets how much chlorine you need to shock.
• pH should be 7.2 to 7.8. Chlorine works noticeably better at the low end of that range.

Step 2: Raise chlorine to shock level and hold it there

Shock level is about 40% of your CYA, never below 10 ppm. With 50 ppm of stabilizer, that is 20 ppm of free chlorine. The pool shock calculator turns that into an exact amount of liquid chlorine or cal-hypo for your pool.

The step most people skip: hold it there. One dose is rarely enough for a green pool. Test morning and evening and top up to shock level each time until the water is clear.

Liquid chlorine is the best choice here, because it adds nothing else. Dichlor and cal-hypo work, but a week of shocking with them adds a lot of stabilizer or calcium.

Step 3: Brush, run the filter and clean it

• Brush walls and floor daily. Algae clings to surfaces where chlorine reaches it slowly.
• Run the pump 24 hours a day until the water is clear.
• Clean or backwash the filter often. Dead algae clogs it fast, and a clogged filter means slow circulation.

The water usually goes green, then gray-blue and cloudy, then clear. Cloudy is progress.

Step 4: Know when you’re done

You’re done when three things are true:

1. The water is clear and you can see the main drain.
2. Free chlorine drops by 1 ppm or less overnight, measured at dusk and again at dawn.
3. A test shows no combined chlorine left (0.5 ppm or less).

Then let chlorine fall back to your normal level. Find that level in the pool chemical calculator.

If the green is metals, not algae

Metals don’t die with chlorine; shocking can make them worse. Signs: the water is green but clear, there is little or no slime, and stains appear on the plaster. Lower pH to about 7.2, add a metal sequestrant, and find the source. Stop using copper algaecide, and have a heater checked for corrosion.

How to keep it from coming back

• Keep free chlorine at or above 7.5% of your CYA, every day.
• Keep CYA at 30–50 ppm in a chlorine pool, 60–80 in a salt pool. Above that, chlorine needs climb fast and the only fix is replacing water.
• Run the pump long enough to filter all the water at least once a day. The pool pump calculator gives you the hours.