Green but My Chemical Levels Look Fine?
You're seeing: Algae or cloudiness, but your test (or the pool store) says chlorine is fine.
Your chlorine is almost certainly too low for your CYA — a reading of 3 ppm only just meets the minimum at CYA 40 and is less than half the minimum at CYA 90, because CYA locks most chlorine into reserve. Get an accurate CYA number with a real kit, compare your FC to the minimum for that CYA, and SLAM. Strips and store tests hide this.
This is the most common pool mystery there is: the water is green or cloudy, but your test — or the pool store — says the chlorine is “fine.” It almost always comes down to one thing.
The short answer: your chlorine is too low for your CYA
Free chlorine isn’t a fixed target. CYA (stabilizer) binds most of your chlorine into a protected reserve, leaving only a small active fraction to actually kill things. The more CYA, the higher your FC has to be to keep the same killing power.
So a reading of 3 ppm free chlorine is:
- Above the minimum at CYA 30 (minimum there is 2, target 4–6), but
- Below the minimum at CYA 80–100 (minimum there is 6–8).
Those are the chlorine-pool numbers. A salt pool’s chart runs lower, with a minimum of 4 at CYA 80, and 3 is still under it.
Same “3,” completely different pool. If your CYA has crept up (usually from tablets), your “normal” chlorine has quietly fallen below the level that keeps algae out — and it grows. See the full FC/CYA chart.
Is your "fine" chlorine actually under the floor?
Enter your CYA and your current FC — if the verdict says you're below the minimum, you've found your green pool.
Don't know your gallons? Work it out here →
Exact geometry at 7.48 gal/ft³. Kidney shapes, diving hoppers and salt/turnover figures live in the full Pool Volume Calculator.
Straight off Trouble Free Pool's FC/CYA chart (minimum ≈ 7.5% of CYA, SLAM ≈ 40%), snapped to the nearest published row — the chart table stays the source of truth. Doses assume 12.5% liquid chlorine; for cal-hypo, dichlor or trichlor use the chemical calculator, which runs the same math. CYA way over 100? More chlorine isn't the fix — lowering CYA is.
Why your test says everything’s fine
Test strips and store tests hide this two ways: they’re imprecise, and some multi-way strips carry a total chlorine pad (free + combined) beside the free one, so a pool full of chloramines can look “chlorinated” on that pad while true free chlorine is near zero. Pool-store computer readouts are also inconsistent on CYA — the same water can read very differently store to store, and that’s the exact number you need to be right.
Do this
- Get an accurate CYA with a real reagent kit — not a strip, not the store (why those two lie).
- Compare your free chlorine to the minimum for that CYA (about 7.5% of CYA on the chlorine chart). If FC is below it, that’s your answer.
- Confirm with the overnight loss test: measure FC after sundown and again at sunrise. More than 1.0 ppm lost overnight = something is eating it.
- If CYA is over 70, partially drain and refill first (see how to lower CYA). Trouble Free Pool’s SLAM starts there for every pool, salt included, even though our salt band runs to 80. Skip it and the level you would have to hold climbs to 31 ppm at CYA 80 and 39 at CYA 100, and your everyday chlorine stays punishingly high until CYA comes down.
- Run the SLAM process at the SLAM level for your CYA until it passes all three exit criteria.
The one thing worth buying here
This entire diagnosis rests on two numbers being right, and the test that told you “your levels are fine” is the reason you’re on this page. Strips are imprecise, and a total-chlorine pad can make a pool full of chloramines read as chlorinated while free chlorine sits near zero — and store CYA readings are inconsistent enough that owners who split one jar of water across two or three stores on the same day report answers as much as 100 ppm apart.
You need a drop-based kit that reads free chlorine separately from combined, and CYA. That rules out the cheaper Taylor K-1004, which doesn’t test CYA at all.
The fix here is almost never a new bottle from the shelf. It’s the right free-chlorine level for the CYA you actually have — which is the whole less-is-more idea in one problem. The kit is the exception that proves it: you buy the measurement once, and it stops you buying everything else.
Frequently asked
Why is my pool green when the chlorine level is normal?
Almost always because your chlorine is too low for your CYA (stabilizer), and that usually means CYA has crept up. CYA binds most free chlorine into a reserve; the higher the CYA, the more FC you need for the same active chlorine. A reading of 3 ppm clears the minimum at CYA 30 but sits below it at CYA 80–100, so algae grows. Test your actual CYA, then compare your FC to the minimum for that number.
Can test strips be wrong about chlorine?
Yes. Strips are imprecise, and some multi-way strips carry a total-chlorine pad (free + combined) beside the free-chlorine pad, so a pool full of chloramines can look chlorinated on that pad when true free chlorine is near zero. Pool-store computer tests are also inconsistent on CYA — the same water can read very differently from store to store. Use a drop-based FAS-DPD kit and an accurate CYA test before trusting any 'your levels are fine.'
How do I confirm my chlorine is actually working?
Run the overnight chlorine loss test (OCLT): measure FC after sundown and again before sunrise. If it drops more than 1.0 ppm with no sunlight to blame, something organic (algae) is eating it — your chlorine is losing the fight regardless of the daytime number. Then SLAM.