Cyanuric acid (CYA) is the one pool chemical you can't fix with another chemical. When it's too high, chlorine goes sluggish and the pool fights you at every step — here's why it happens and exactly how to bring it down.
Why you can only dilute CYA
CYA — "stabilizer" or "conditioner" — shields chlorine from sunlight, but it doesn't get used up and nothing on the market removes it. The only ways down are replacing water (drain & refill) or a reverse-osmosis mobile service that filters it out.
What counts as too high
Over about 100 ppm for a traditional chlorine pool (or ~100–120 for saltwater) is the trouble zone. The more CYA you have, the more free chlorine you need just to keep the same killing power — which is why over-stabilized pools cloud up or go green even with a "normal" chlorine reading.
How it got so high
Almost always from stabilized chlorine: trichlor tabs and dichlor shock each add CYA every time you use them. It accumulates quietly over a season until chlorine stops working. Switching to liquid chlorine (which adds none) is how you keep it from climbing again.
How to lower it — the dilution math
Draining and refilling a percentage of the water drops CYA proportionally. Enter your current and target level and we'll tell you exactly how much to replace. Lower-CYA calculator →
Do it in stages on a hot, dry day so the pool refills quickly, or book a reverse-osmosis service to skip draining altogether.
Safety first: never fully drain an in-ground pool without checking your local water table — an empty shell can float out of the ground. A partial drain is almost always the safer route.
After you lower it, reset your chlorine
With CYA back in range (30–50 for chlorine pools, 60–80 for salt), set free chlorine to about 7.5% of your CYA. CYA-scaled chlorine calculator → If the pool already turned green from weak chlorine, run the SLAM recovery →. New to balancing? Start with the right order →.