Keep CYA between 30 and 50 ppm.

30 to 50 parts per million is the sweet spot for cyanuric acid in an outdoor chlorine pool.

CYA is sunblock for chlorine. Without it, UV rays burn off your chlorine in hours. With too much, chlorine gets locked up and stops working effectively. The 30–50 range gives you protection without over-stabilizing.

If you live in a scorching climate, some pool pros push it to 60 or 70. That can work, but you’ll need to keep free chlorine higher to compensate. Most pools do fine at 40. Test monthly, because CYA doesn’t evaporate — it only goes down when you drain or backwash.

Don’t let CYA creep up. High CYA is the most common pool chemistry mistake, and the only fix is dilution.