Cyanuric Acid: The Most Misunderstood Pool Chemical

Cyanuric acid — CYA, or "stabilizer" — is the chemical that trips up more pool owners than any other. Get it right and your chlorine lasts all day. Get it wrong in either direction and your pool fights you constantly. Here's what it actually does, and why more is definitely not better.

What it does: sunscreen for your chlorine

Unprotected chlorine in an outdoor pool is destroyed by UV light — a sunny afternoon can burn off most of it in hours. CYA binds to chlorine and shields it from the sun, so it survives long enough to do its job. Without any stabilizer, you'd be adding chlorine constantly and still testing zero by evening — which is exactly what the overnight chlorine loss test helps you rule out.

Too little: your chlorine vanishes

Below about 30 ppm CYA in an outdoor pool, sunlight strips chlorine faster than you can keep up. If you're dosing correctly with the chlorine calculator but the level still crashes every day, low stabilizer is the usual culprit — check it before you blame anything else. Raising it is simple with the CYA calculator.

Too much: your chlorine goes to sleep

Here's the part nobody explains: high CYA doesn't just protect chlorine, itweakens it. The more stabilizer in the water, the more chlorine you need to get the same sanitizing punch. Push CYA past ~80–100 ppm and your normal chlorine level effectively stops working — the water can test "sanitized" and still go cloudy or green. This is the classic trap of relying on trichlor tablets, which add stabilizer with every puck until it stacks up.

The FC/CYA ratio is the whole game

Chlorine and stabilizer aren't two separate readings — they're a ratio. A good rule keeps your free chlorine (FC) around 7.5% of your CYAlevel. So 40 ppm CYA wants ~3 ppm FC; 80 ppm CYA wants ~6 ppm. Testing chlorine without knowing CYA is how pools end up "sanitized on paper" but algae in practice. If yours has already turned, the green pool calculator sizes the fix around this exact ratio.

The only way to lower it

Unlike almost every other pool chemical, there's no additive that removes CYA. The only practical way down is to drain and replace water— dilution. That's why "more is better" is such an expensive mistake here: overshoot and your only fix is a partial drain. Add stabilizer slowly, retest after a few days, and aim for the low end of your range. The full picture of how CYA fits with the other numbers is in Pool Chemistry 101.