Cyanuric acid (CYA)
Also called: CYA, stabilizer.
Cyanuric acid (CYA), sold as stabilizer, is sunscreen for chlorine: without some in the water, UV burns off chlorine in hours. Cyanuric acid also binds most of the free chlorine into a reserve, so the higher the CYA, the more free chlorine a pool needs.
Why it matters
CYA only goes up when you add it, and trichlor tablets and dichlor granules are about 57% cyanuric acid by weight, so every dose adds more. It does not evaporate or break down. It only leaves by dilution: replacing about half the water cuts CYA roughly in half, and CYA reducer products are unreliable. Hold it at 30–50 in a chlorine pool and 60–80 in a salt pool. Over 100 ppm it is a real problem in any pool. Test it with a drop kit, because pool-store CYA readings are inconsistent store to store.
In numbers
A free chlorine reading of 3 ppm is fine at CYA 40 but near-useless at CYA 90.