PooloraHow to fix a green pool
Green water is algae that grew because chlorine fell too low for your stabilizer. The fix is simple but needs patience: raise chlorine and keep it there until three tests say you are done.
- Test free chlorine, pH and stabilizer (CYA) before adding anything.
- If CYA is above 25 ppm, replace part of the water first: the 10 ppm label limit is not enough above that.
- Raise chlorine to 10 ppm and keep it there until the water is clear, combined chlorine is 0.4 ppm or less and the pool loses 1 ppm or less overnight.
Why pools turn green
Algae is always trying to grow. Free chlorine keeps it away only while it stays above a minimum, and that minimum rises with your stabilizer (cyanuric acid, CYA), because stabilizer holds on to part of the chlorine. A few days below the line, especially in heat and sun, is all algae needs.
| Stabilizer (CYA) | Minimum free chlorine |
|---|---|
| 0 ppm | 1 ppm |
| 20 ppm | 2 ppm |
| 30 ppm | 2.25 ppm |
| 40 ppm | 3 ppm |
| 50 ppm | 3.75 ppm |
| 60 ppm | 4.5 ppm |
| 80 ppm | 6 ppm |
So "my chlorine was 2 ppm" can still mean a green pool if stabilizer is 60. That is also why the cure depends on stabilizer.
Step 1: test before you buy anything
Test free chlorine, pH and stabilizer, ideally with a drop kit (the CDC notes strips are less accurate). Very high chlorine can bleach a DPD test to zero; if the reading makes no sense, dilute the sample with distilled water and test again.
Step 2: check stabilizer against the label limit
Chemistry says algae clears when free chlorine is held around 40% of your CYA. But the chlorine labels we checked cap superchlorination at 10 ppm, and 10 ppm is 40% of 25. So:
- CYA 25 ppm or less: shock to 10 ppm (step 4).
- CYA above 25 ppm: 10 ppm will not be enough. Lower stabilizer first by replacing water.
| Your CYA | Water to replace to reach 25 ppm |
|---|---|
| 30 ppm | 17% |
| 40 ppm | 38% |
| 50 ppm | 50% |
| 70 ppm | 64% |
Refill with water that has no stabilizer. Some pools should not be fully drained (vinyl liners, fiberglass and pools in high groundwater can be damaged); replace water in steps and follow your builder's guidance.
Step 3: clean and filter
Scoop out leaves and debris, brush walls and floor, and run the pump around the clock until the job is finished. Clean or backwash the filter when its pressure climbs; a green pool clogs filters fast.
Step 4: raise chlorine to 10 ppm and hold it
Amounts to reach 10 ppm in 10,000 gallons (for other sizes, see the pool chemical charts by size):
| Current FC | Liquid chlorine 10% | Liquid chlorine 12.5% | Bleach 6% | Cal-hypo 65% |
|---|---|---|---|---|
| 1 ppm | 14.5 cups 3.4 L | 11.5 cups 2.75 L | 1.5 gal 5.5 L | 1.2 lb 525 g |
| 2 ppm | 12.75 cups 3.05 L | 10.25 cups 2.4 L | 1.3 gal 4.9 L | 1 lb 465 g |
| 3 ppm | 11.25 cups 2.65 L | 9 cups 2.1 L | 1.1 gal 4.3 L | 14.5 oz 410 g |
Example: a 15,000-gallon pool at 0.5 ppm needs about 1.4 gal (182 fl oz · 5.4 L) of 10% liquid chlorine to reach 10 ppm.
Algae burns chlorine fast, so test several times a day and add back up to 10 ppm each time. Holding the level steady is what kills algae; one big dose that drops to zero by morning does not. Liquid chlorine or bleach is the best choice here: cal-hypo adds calcium (about 7.1 ppm per 10 ppm of chlorine) and dichlor or trichlor add stabilizer, which is the last thing a green pool needs.
Step 5: know when you are done
Stop shocking only when all three are true:
- The water is clear: you can see the main drain.
- Combined chlorine is 0.4 ppm or less (the ANSI/APSP/ICC-11 limit).
- The pool loses 1 ppm of free chlorine or less overnight. Test after sunset and again before sunrise.
Then let chlorine drop back into the swimming range printed on your label (often up to 3 or 4 ppm) before anyone gets in, and keep it above the minimum for your stabilizer from now on.
Common questions
How long does it take?
It depends on how much algae there is and how steadily chlorine is held at 10 ppm. A light green pool can clear in a few days; a dark one takes longer. Letting chlorine drop between doses is what makes it drag on.
Can I use algaecide instead?
Algaecide can help prevent algae, but chlorine is what kills an active bloom. Some algaecides contain copper, which can stain plaster. Fix the chlorine level first.
It stopped being green but it is cloudy. Now what?
That is dead algae and fine debris. Keep chlorine up and keep filtering; see the cloudy pool guide.
Do it with your own products
Poolora turns your test into the exact amount of the products on your shelf, in cups and ounces, in the right order, and tells you when it's safe to swim. Free to start; every chemical, water balance and history are part of Pro.
Keep going
Sources
- CDC: Home pool and hot tub water treatment and testing (minimum chlorine; DPD vs strips)
- U.S. EPA registered labels for sodium hypochlorite pool products (Reg. No. 70271-10 and 82357-1): swimming re-entry range and 5–10 ppm superchlorination
- O'Brien, J. E. et al. (1974), chlorine and cyanuric acid equilibria in water
- ANSI/APSP/ICC-11 (combined chlorine 0.4 ppm or less)
- Doses: Poolora engine (mass balance for chlorine; carbonate-equilibrium model for pH with constants from the USGS PHREEQC database)