Cyanuric acid (CYA) — also called “stabilizer” or “conditioner” — is the sunscreen for your chlorine. Without CYA, a Florida outdoor pool loses 75–90% of its free chlorine to UV in a single day. With the right amount of CYA, chlorine's half-life in sunlight extends to days.
Target range: 30 – 50 ppm(outdoor pools). Salt pools often run 60–80 ppm for cell efficiency. Indoor pools: zero — you don't need it.
How CYA works
CYA binds loosely with free chlorine, creating a reservoir that slowly releases active chlorine as UV destroys the unbound molecules. The bound chlorine is still sanitizing, just at a lower rate — which is why the FC/CYA ratio matters more than either number alone.
The CYA lock problem
Here's where pool owners get stuck. CYA only leaves the water through dilution — draining or splashout. It doesn't break down, doesn't evaporate, doesn't filter out. Every tab of trichlor you add contains CYA, so it just keeps climbing.
When CYA hits ~100 ppm or higher, chlorine becomes so bound up that normal 3 ppm FC essentially does nothing. Algae thrives. Shock treatments fail. This is “CYA lock” or “chlorine lock.”
How to lower CYA
There are only three real options:
- Partial drain and refill. Replacing 50% of water cuts CYA in half. Most common fix.
- Reverse osmosis (RO) service. A truck pulls up, filters your water, and returns it CYA-free. Expensive but preserves the water (huge in Florida).
- Wait for splashout and evaporation. Slow. Can take a full season.
How to raise CYA
Add granular stabilizer through the skimmer with pump running. Do notbroadcast it — it takes days to dissolve and can bleach the finish. Test CYA one week after adding; it doesn't register immediately.
Avoiding the CYA creep
If you use trichlor tablets(which contain ~57% CYA by weight), you're adding ~6 ppm CYA for every 10 ppm of chlorine you use. Switch to liquid chlorine (sodium hypochlorite) or a salt chlorine generator — both add zero CYA.