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 yield ~55% CYA by mass after the chlorine is consumed), 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.
Pool CYA / stabilizer calculator
Enter pool size, current CYA, and pool type. Math: 1 lb per 10,000 gallons raises CYA 12 ppm.
From a turbidity (black-dot) test — strips read CYA poorly.
Add ≈ 2.5 lbs of cyanuric acid
Raises CYA by 20 ppm in 15,000 gallons. Use the sock method: granules in an old sock, hung in front of a return jet — CYA dissolves slowly (up to a week) and dumping it loose etches plaster. Don't retest for 3–7 days.
Higher CYA needs higher free chlorine to sanitize the same water — don't chase a big number. More stabilizer is not more protection.
Frequently asked questions
No. CYA only leaves the water through dilution, meaning draining or splashout. It doesn't break down, doesn't evaporate, and doesn't filter out. That's exactly why it keeps climbing on you, especially if you're running trichlor tabs, since every tab you drop in carries more CYA with it.
That's CYA lock. When cyanuric acid climbs to around 100 ppm or higher, your chlorine gets so bound up that a normal 3 ppm free chlorine reading essentially does nothing. Algae thrives, shock treatments fail, and the test looks fine while the water goes bad. Check your CYA before you keep dumping chlorine at it.
A partial drain and refill. Replacing 50 percent of the water cuts CYA in half, and it's the most common fix. Reverse osmosis service is the other real option, where a truck filters your water and returns it CYA-free without wasting it, which matters down here. Waiting on splashout works too but can take a full season.
Add granular stabilizer through the skimmer with the pump running. Do not broadcast it across the pool. It takes days to dissolve and sitting on the plaster it can bleach the finish. Then test CYA about a week after adding, because it doesn't register right away.
Usually, yeah. Outdoor pools target 30 to 50 ppm, but salt pools often run 60 to 80 ppm because it helps the cell run efficiently. Indoor pools are the opposite. They need zero, since there's no UV to protect the chlorine from in the first place.
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