Diagnose my pool: what's wrong, and how to fix it
Something's off and you want the answer, not a chemistry lecture. Tell us what you're seeing and the couple of numbers that matter — we'll hand you the diagnosis, the exact target, and the fix, then link the deeper guide if you want it. Every number here uses the same FC/CYA chart our guides do, so the tool and the advice never disagree.
Diagnose your pool
Tell us what you're seeing and the couple of numbers that matter. We'll hand you the diagnosis, the exact target, and the fix — not a forum thread to wade through.
👆 Pick what you're seeing to get your diagnosis.
Targets use the same FC/CYA chart as the 5 numbers — minimum ≈ 7.5% of CYA, SLAM ≈ 40% of CYA. Not sure of a number yet? Test it right first (strips lie). Just want the targets? The 5 numbers ·exact doses in your gallons.
How this works (and why it's not a forum)
Most pool advice makes you read a methodology, then self-diagnose across a dozen threads. We flipped it: pick a symptom, and the tool asks only for the numbers that actually decide that problem, then computes your answer — your SLAM level from your CYA, the free-chlorine minimum you're probably under, the exact share of water to drain when CYA is too high. It's the difference between "here's how pool chemistry works" and "here's what to do right now." When you want the full story, every result links the guide underneath it.
Prefer to just ask? Browse all of Pool School → ·See the 5 numbers & FC/CYA chart → ·Get exact doses in your gallons →
Common pool problems, answered
Why is my pool green even though I add chlorine?
Almost always because your free chlorine is too low for your CYA (stabilizer) — not because you added none. CYA locks most chlorine into reserve, so the higher your CYA, the more free chlorine you need for the same killing power. If CYA crept up (usually from tablets), your "normal" chlorine quietly fell below what algae needs to die. The fix is to SLAM: raise free chlorine to about 40% of your CYA and hold it until the water clears, combined chlorine is ≤ 0.5, and it passes an overnight loss test. If CYA is 90+, partially drain first.
Why won't my chlorine hold or stay up?
Two usual causes. First, your free chlorine is below the minimum for your CYA (about 7.5% of CYA), so it is being consumed faster than it sanitizes — raise it to target. Second, your CYA is very high, which makes the required chlorine climb until it feels like it never holds; the only fix there is to dilute (partial drain). Confirm with an overnight chlorine-loss test: measure free chlorine at dusk and dawn — losing more than 1.0 ppm overnight means something organic is eating it.
What is "chlorine lock" and how do I fix it?
There is no such thing as chlorine lock — it is a myth. What pool stores call chlorine lock is over-stabilized water: your CYA is too high, so your normal free chlorine is below what that CYA requires and no amount of shock "unlocks" it. The only reliable fix is to lower CYA by dilution — partially drain and refill (roughly proportional: replace about half the water to cut CYA about in half) or use a reverse-osmosis service. "CYA reducer" products are unreliable.
Why does my pool smell strongly of chlorine and sting my eyes?
Counterintuitively, that means too little free chlorine, not too much. The harsh smell and burning eyes come from chloramines (combined chlorine) — chlorine that already reacted with sweat, sunscreen and oils and got used up. They build when free chlorine is too low to burn through them. Test combined chlorine; if it is above 0.5, add chlorine (SLAM) to clear it. A properly chlorinated pool barely smells at all.
Why is my pool cloudy but not green?
Cloudy-but-not-green is almost never algae, so do not panic-SLAM it. It is usually one of three things: high pH or calcium (the water cannot hold it, so it clouds), a filtration shortfall (too few pump hours or a dirty filter), or chloramines from chlorine running too low. Check in that order — pH and calcium first, then combined chlorine, then the filter. Skip the clarifier; it hides the haze instead of fixing the cause.
Pool School is free and independent. When a fix genuinely needs a product — a real FAS-DPD test kit, a robot that keeps up — we'll point you to the honest pick in a showdown, and we say so plainly. We never sell you a bottle you don't need.