My CYA was at 80–confirmed by 2 separate tests. My SLAM took nearly 2 weeks and a lot of bleach to sustain a shock level of 31ppm (also confirming the CYA Reading was correct). After successful SLAM and OCLT I opted to reduce my CYA to lower my target FC and spend on bleach. I drained about 8” off a 22,000 gallon pool and refilled. I figured my new CYA would be somewhere in the 50-70 range as there is no way I drained more than 3-4K gallons. CYA is barely registering 20–I can still see the dot. Reagents are fresh and using. a TF-100 kit. And I know I’m doing the test correctly. What could possible account for almost no CYA after a 10-15% water replacement on a CYA of 80. Anyone have any ideas?