Don't waste money on CYA reducers. Drain and refill.

They don’t work well enough to be worth the hassle or cost. CYA doesn’t break down naturally on its own, so the only reliable way to lower it is to remove the water that contains it. Reverse osmosis trucks can work, but they’re expensive and not available everywhere.

The reality is that high CYA makes your chlorine basically useless. You can keep dumping shock in, but it won’t do much. The pool store will sell you a bottle of “CYA reducer” — it’s a bacteria that supposedly eats the cyanuric acid. In practice, it’s slow, finicky, and might knock it down a bit, but not enough to fix a truly high level.

If you absolutely cannot drain (HOA rules, well water concerns, or you have a liner that might collapse), you can do a partial drain — say 25–50% — and refill. That’s still draining, but less water overall. Anything short of that is a gamble.

Draining is the fix. Everything else is a temporary patch that usually ends with you draining anyway.