Partial drain is the only reliable way to lower cyanuric acid.

Partial drain and refill. That’s it. You have to physically remove water from the pool and replace it with fresh water.

CYA doesn’t go away on its own. It doesn’t evaporate, doesn’t break down from sunlight, and no chemical additive actually destroys it despite what the bottle claims. If your CYA is over 80–100 ppm (or 50 for saltwater pools), you’re stuck diluting.

How much to drain? Simple math: If your CYA is 120 and you want it at 40, you need to replace about 2/3 of the water (because 40/120 = 1/3 remaining). So drain 66%, refill, retest. It’s annoying, and it costs water, but it works in one afternoon.

The smarter move is to not let it get that high in the first place. Stop using stabilized chlorine tabs (trichlor) or shock that contains CYA. Switch to liquid chlorine or cal-hypo for daily sanitation, and only add CYA intentionally to a target of 30–50 ppm.

Future you will thank you for not having to drain half the pool again.