Cyanuric acid protects your chlorine from the sun — and quietly holds most of it hostage. Get the ratio wrong and a pool can read “perfect” and still grow algae. Here’s the real science, and the health-department limits behind it.
of the chlorine in a typical CYA-stabilized pool is actually active sanitizer at any given moment. The other ~97% is held in reserve, bound to cyanuric acid — which is exactly why the ratio matters more than the chlorine number alone.
CYA (also called stabilizer or conditioner) and chlorine aren’t two separate dials — they’re chemically tied together. Here’s how.
Ultraviolet light tears free chlorine apart. In open Florida sun, an unstabilized pool can lose 75–90% of its chlorine in a couple of hours. Cyanuric acid (CYA) acts like sunscreen — it shields chlorine from UV so it survives the day.
CYA doesn’t neutralize chlorine — it holds it. Most of your free chlorine binds reversibly to CYA as chlorinated isocyanurates, forming a reservoir that slowly releases active chlorine back into the water as it gets used up.
The real sanitizer is hypochlorous acid (HOCl). At any instant only a tiny fraction of your free chlorine is unbound, active HOCl — the rest is parked on CYA. That reserve is good… until there’s so much CYA that almost nothing stays active.
More CYA means more chlorine gets bound up, so you need a higher chlorine reading to keep the same killing power. That’s the whole correlation: chlorine and CYA have to move together, in ratio — not independently.
The active killer is hypochlorous acid (HOCl). Cyanuric acid reversibly binds it — HOCl + CYA ⇌ chlorinated isocyanurate — pulling most of it out of play and feeding it back slowly. Raise CYA and the equilibrium shifts: less active HOCl for the same chlorine reading.
The rule pros use: keep free chlorine around 7.5% of your CYA, and never below ~5%. Slide your CYA and watch the chlorine you actually need.
A guideline, not a prescription — salt pools, indoor pools, and active algae all shift the numbers.
| CYA (ppm) | Min FC | Target FC | Status |
|---|---|---|---|
| 30 | 1.5 | 2–3 | Ideal |
| 40 | 2 | 3 | Ideal |
| 50 | 2.5 | 4 | Good |
| 60 | 3 | 4–5 | Getting high |
| 80 | 4 | 6 | Hard to hold |
| 100 | 5 | 7–8 | Health-code max |
| 120 | 6+ | 9+ | Chlorine lock — dilute |
Stabilizer only goes up — every stabilized chlorine tablet (“trichlor”) adds more. Once it’s too high, no amount of chlorine is practical. Watch for:
The only real fix is dilution — partly draining and refilling to bring CYA back into range. It comes down the same way nitrates do. See our nitrates guide →
This isn’t just about clear water — it’s about whether the water is actually killing what’s in it. That’s why CYA is written into pool code.
The CDC’s Model Aquatic Health Code recommends keeping CYA at or below 90 ppm, and Florida’s public-pool code (64E-9) caps it at 100 ppm. The reason is disinfection: as CYA climbs, the same chlorine reading kills germs far more slowly.
High CYA increases the contact time chlorine needs to inactivate bacteria and parasites. Cryptosporidium is the worst case — the CDC notes CYA dramatically lengthens the time (and chlorine level) required to kill it, which is why hyperchlorination guidance says to lower CYA first.
A backyard pool isn’t inspected like a public one, but the chemistry is identical. Keeping CYA in range and chlorine in ratio is exactly how you keep a private pool genuinely sanitary — not just clear-looking.
Sources: CDC Model Aquatic Health Code (MAHC); Florida Administrative Code 64E-9 (public swimming pools); CDC guidance on cyanuric acid and Cryptosporidium inactivation. Limits cited are for public pools; private-pool chemistry is identical but unregulated.
The questions homeowners ask most about stabilizer. See all FAQs →
For most chlorine pools, 30–50 ppm is the sweet spot — enough UV protection without choking your chlorine’s strength. Saltwater pools often run a bit higher (60–80 ppm) because the cell adds chlorine continuously. Above 100 ppm you’re into health-code and chlorine-lock territory.
Almost always high CYA. If your CYA is very high, a “normal” chlorine reading of 3 ppm leaves too little active hypochlorous acid to sanitize, so algae grows in water that looks chemically fine. It’s the ratio that matters, not the chlorine number alone.
There’s no chemical that removes CYA reliably. The only practical fixes are dilution — partially draining and refilling with fresh water — or reverse-osmosis filtration. It comes down the same way nitrates do: you replace water.
At normal pool levels CYA itself isn’t considered a swimmer hazard. The real risk is indirect: too much CYA weakens chlorine’s ability to kill germs, so the water can be under-sanitized even when it looks clear. That’s exactly why health departments cap it.
We test stabilizer and balance your chlorine to match on every visit — so your pool is genuinely sanitized, not just clear. Questions? Reach us anytime at (727) 295-3621.
Got a full-sun pool out on the barrier island? See how we handle pool service in St. Pete Beach, where stabilizer is the whole ballgame.