I agree with the temperature. I would try to keep it around 76 and see if that helps. How old are the danios? I lost a whole school and it turned out that my angel fish was chasing them to death. By the time I figured out what was wrong, it was too late.
The way to help them recover from stress enduced "waisting" is a lower temperature.
I'm sure you're feeding them enough - danios generally have NO problem eating...
I'm not sure what is going on in your case BUT lower that temp and see if it helps. When their immune system becomes weakened by *whatever* sometimes they just can't recover. .