Partial drain and refill is the only fix.

Yep. CYA doesn’t break down on its own, and no chemical is going to fix it in a reasonable timeframe. You have to dilute.

How much to drain depends on how high your CYA is and what level you’re targeting. Most pools aim for 30-50 ppm. If you’re at 150, you’re looking at swapping out about 75-80% of the water. That might mean multiple partial drains over a few days — don’t drain a concrete pool completely or you risk floating the gunite.

There are “CYA-lowering” bacteria treatments, but they’re slow (weeks) and unpredictable. Not worth the hassle for most people. Just drain, refill, and retest. Future you will thank current you for not over-stabilizing with trichlor tablets.