What is the only reliable way to lower cyanuric acid (CYA) in a pool?

Partial drain and refill is the only reliable way to lower CYA. CYA doesn’t evaporate or break down easily, and no chemical or filter removes it. Replace 50–70% of the water if CYA is over 100 ppm, aiming for 30–50 ppm. Do it in stages to avoid ground pressure or liner damage.