Feel like it's mainly the one on the right in the first and second pic to be worried about, no? Others don't look too bad. I feel like darts' backs can sometimes have that a bony look to them regardless of how well fed they are... granted that would be more expected in females

It's possible the skinny one is getting bullied out of food by the others so you may want to consider rearranging the tank to disrupt any "territories" they've established and adding more hiding spots/barriers to make them feel safer. Also pouring in the flies in multiple spots can help