After discovering that I had a mustard algae problem ("hey, why does this algae keep showing up only in the shady areas even after I brush it off and keep FC good?"), I started the recommended SLAM procedure using levels for mustard algae. When I started the SLAM my CYA was 40, and it has been 40 since I got my house. After 24 hours of SLAM levels my overnight FC loss was less than 1 mg/L and the pool was clear, but I thought I'd run it another 24 to be safe. The next day (the 2nd SLAM day), I noticed that I'd lost about 8 mg/L of FC and it was only noonish. I added some more to bring it back up. Same thing the next night - FC loss under 1 mg/L, but when I tested in the afternoon I had lost over 10 mg/L FC.
Now I'm scratching my head and thinking, "why isn't the CYA protecting the chlorine?" So I do a CYA test. ZERO! Bubkiss, notta, zilch. I run another one. Same result. I fill the tube to the top and I can still make out the black dot. It's very faint, but I can make it out.
So, assuming that I didn't accidentally discover that you can destroy CYA with 20+ mg/L FC, what could be going on? Do high levels of HClO affect the CYA test?