CYA reducers work, but draining is faster.

You can, but it’s a pain.

There are enzyme-based CYA reducers that break down cyanuric acid over a few weeks. They work—eventually—but you have to follow the directions exactly, keep your chlorine level low, and wait. I’ve used them. They’re not magic, and they’re not cheap.

For really high CYA (100+), you’re looking at multiple doses and weeks of careful monitoring. The alternative is a reverse osmosis truck, which filters the water without draining. That works fast, but it costs a few hundred bucks and you have to find a service in your area.

If you can drain a third to half the pool and refill, do that. It’s faster, cheaper, and certain. If you cannot drain—say, due to water restrictions or a liner that can’t handle it—then the enzyme route is your next best bet. Just don’t expect overnight results.

Draining still wins on effort and cost.