Is it possible that some other chemical that can't be detected with your test kits is leeching into the water from somewhere? Could be from the drftwood or something else that was added to (or nearby) your tank.
Just a thought.
I lost an entire 29g population once during an annual whole-tank cleaning. I had moved the residents to smaller tanks and cleaned the entire 29g, gravel and all. I had placed the gravel in some 5-gallon buckets in the garage. Pest control ended up spraying chemicals near the buckets (which I did not know at the time). Once I put everything back together and cycled the tank, I moved the fish back. They all died, starting with the bottom feeders and working upward, within a few days. Had water tested later and found pesticides in the water, the kind used by pest control companies. Very sad day for me as I had owned some of the fish for several years. And this was the day I gave up the hobby in my youth . . .
Just a thought.
I lost an entire 29g population once during an annual whole-tank cleaning. I had moved the residents to smaller tanks and cleaned the entire 29g, gravel and all. I had placed the gravel in some 5-gallon buckets in the garage. Pest control ended up spraying chemicals near the buckets (which I did not know at the time). Once I put everything back together and cycled the tank, I moved the fish back. They all died, starting with the bottom feeders and working upward, within a few days. Had water tested later and found pesticides in the water, the kind used by pest control companies. Very sad day for me as I had owned some of the fish for several years. And this was the day I gave up the hobby in my youth . . .