Wow!
Have just read the entire thread just now!
Such a crazy and puzzling event. All my thoughts or idea that may lead to the cause of the deaths of these tetra have been spoken aloud, sealant, aerosol cleaners, chemicals, pH, heating, substrate, rocks etc were offered by the other guys and still no real answer.
Just one last one theory though, you have no plants in the tank now and the tetras are surviving so far.
Something to do with one of the plants perhaps?
Only reason I bring this up as I have read on other threads that some plants are harmful and can cause deaths, mainly to shrimps though.
And wonder if these plants were cleaned with a chemical or something by LFS or distributer prior to you purchasing them?
If its a plant toxin then it would make sense if the toxins floated to the surface of water where tetras and affected them were but the cories are at the bottom of the tank, away from the surface therefore they were unaffected.
Just a theory anyway and probably grasping at straws here
Very doubtful if this would be the cause and don't even know for sure if thats a feasible theory.
Really hope you will find out what caused all this, even if you don't, lets hope it does not occur again.
Keeping my fingers crossed and good luck