From my experience, if there isn't anything wrong with parameters, neon tetras dying after being put into the tank usually has something to do with the quality of fish.
After cycling my first tank, I got 7 neon tetras and not a single one died. I got them from an lfs
a half an hour's drive away, which quarantines all fish before selling.
After that, I tried to give a betta a few companions and made the mistake of buying them from PetSmart. They died a week later and took the betta with them (ich).