What is the best way to lower high cyanuric acid in a pool without draining it?

Reverse osmosis is the only real option for lowering cyanuric acid without draining and refilling. The truck filters water through membranes that strip out CYA and other dissolved solids, then sends clean water back, losing only 15–20% of the water. Enzyme-based reducers are unreliable — best case they lower levels by 20–30 ppm over a week, worst case they do nothing. The downside is cost: $400–600 depending on pool size and location, but that’s cheaper than replacing all the water.