Two weeks ago my CYA tested at 20. I added stabilizer and waited two weeks to re-test. All other tests over that period were typical with CL = 4-5 and ph = 7.4 to 7.6. Then over 2 days (8 days after adding stabilizer) I lost all the chlorine! Ok, I know I need to SLAM, but what would cause the CL to disappear? SWG lights indicate it's operating. The new CYA is 50.