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Any thoughts why my CYA would be reading so low? In late May I added 14lbs. 2 weeks after that, they said it was reading 0, so I added the 2lbs I had left. We do not trust pool store testing, in particular CYA readings. I also was shocking with granular shock, so it should have added more. Yes, it would have. This reinforces the fact that the result was off. Does algae use up/eat away CYA? No. Does another chemical I could have added lower it as well? The only practical way to lower CYA is a water replacement.
I ran the OCLT last night with the spa closed off and it went from 10FC to 7.5FC again. I'm testing the water from the same spot every time to get consistent results, sometimes every 2-3 hours, sometimes every 6 or so (depending on my schedule). I'm brushing at least once a day, sometimes twice (or if I get a weird reading, i brush, wait a few minutes, and re-test but it has still read the same, so I know it's just actually low). I'll adjust my eyeballs today and see if that helps. But again, it doesn't look like I have any algae issues, so it's obviously frustrating to have the test keep telling me something is wrong. Should I clean out the filter before doing the OCLT? Maybe the dead algae on the cartridges are causing problems? Dead algae is dead. It doesn't affect OCLT. Only live algae causes OCLT failure.