Teras can be nippy. They have teeth in the front of their mouths. They are a pirhana breed.
I have found, and they do have a reputation, serpae to be very nippy and to do damage when the mood takes them. I had to remove my serpae from the main community tank. When mine got crowded they got aggressive. They took the eye out of one of their own. All the serpae were bruised and battered with their pretty fins ragged, torn and chomped off.
Some of the black phantoms joined the fray. All the tetra had their tail fins nipped, even some diamond head neons. Fish that stayed out of the way were ok, but it was not peaceful and no one was safe. The schoolers stayed together and the others stayed inside.
I took the serpae out and will either make a small tank with them or return the ones who recover well.
The main factor was that they seem competetive and like their space. They seem to be more aggressive when they are crowded, they are expecting food, and there are a shoal of them. I have two still in the big community tank. Those two aren't a problem any more. Tetras are a nippy fish. The larger ones can do damage. I even see the diamond heads nip, but they are too small to do much damage.
Ilike most of my tetras, but they nip and the larger ones can do damage. And some of them get very cranky.