IME, I used to have serpae tetras(a group of 5 when I was newbie), and then for some reason, all the serpaes except 1 died, and then since it wasn't able to school with my platys, and corys, I found it dead the next morning
I think they die because they are very stressed out without other fish in the tank(that are similar to them, and make them feel safe), and are scared because they can't school or anything, especially during feeding time, they school because 1 at a time, one fish darts up to get food, and comes back down.
Good Luck
sorry for your loss