Here's a conversation we have at least once a month, usually next to a cloudy-green pool:
"I don't get it. I keep chlorine in it constantly. The test strip says it's fine. Why does it keep going green?"
Nine times out of ten, the answer is a chemical most pool owners have never tested: cyanuric acid, or CYA, also sold as "stabilizer" or "conditioner." It's simultaneously essential in Florida and the most common root cause of chronic pool problems we see. Both things are true, and the difference is entirely about the number.
What CYA does (the good part)
Chlorine has a fatal weakness: sunlight. UV radiation destroys free chlorine fast — in unprotected water under a Florida summer sun, you can lose half your chlorine in under an hour. CYA molecules bond loosely with chlorine and shield it from UV. With a moderate CYA level, chlorine survives all day instead of burning off by lunch.
That's why CYA around 30–50 ppm is the sweet spot for Florida pools. Below that, you're pouring chlorine into a UV incinerator.
What CYA does (the problem part)
The same bonding that protects chlorine also slows it down. Chlorine bound to CYA is dramatically less active against algae and bacteria. At moderate CYA levels this is a fine trade. But as CYA climbs, more and more of your chlorine is locked up at any given moment.
At 100+ ppm of CYA, a pool can test "fine" for total chlorine while the active chlorine — the part actually killing things — is close to zero. The test strip says you're protected. The algae knows better.
Why CYA creeps up on Florida pools
Here's the mechanism nobody explains at the pool store: the most popular chlorine product adds CYA every time you use it.
Trichlor tablets — the pucks in your floater — are about half stabilizer by weight. Every tablet dissolves into chlorine (which gets used up) and CYA (which doesn't). CYA essentially never leaves the water on its own; it doesn't evaporate, and it doesn't get consumed. It just accumulates, tablet after tablet, month after month.
A pool running on trichlor pucks can add 5–10 ppm of CYA per month. Within a couple of years, you're at 150 ppm and wondering why "nothing works anymore."
The classic Florida death spiral
- Pool gets a floater full of tablets. Works great — for the first year.
- CYA quietly climbs past 80, 100, 120 ppm.
- Chlorine becomes less and less effective. Algae blooms start.
- Owner responds with more tablets — adding more CYA.
- Pool store sells a copper-based algaecide, which masks the problem (and can stain).
- Pool is now chronically green-ish with "perfect" test numbers.
If that sequence sounds familiar, your pool doesn't need more chlorine. It needs a CYA test and probably a partial drain.
The fix
Test it. Any decent pool service should test CYA routinely (we do, every week). If you DIY, a proper test kit — not basic strips — is worth the money.
If it's high, dilute. CYA doesn't degrade; the only practical fix is replacing water. A partial drain-and-refill — often a third to a half of the pool — brings it back into range. In Florida, drain carefully and never fully: high groundwater can lift a pool shell out of the ground. When in doubt, get help.
Change the chlorine source. After correcting, switch off straight trichlor dependence — liquid chlorine adds no CYA. That's how we run our service routes: stabilizer stays where we put it.
The takeaway number
Keep CYA between 30 and 50 ppm, test it a few times a year, and be suspicious of any "add more tablets" advice that doesn't come with a CYA reading. If your pool has been chronically green despite good numbers, this is the number nobody checked — and if it's already a swamp, our green-to-clean service tests it on the first visit.