Red Eye Tetra Mouth Stuck Open

[Schmoo]

I have 3 red eye tetra that I've had for quite awhile and were used to cycle a tank. They are healthy and have grown to a pretty good size. I just noticed that one of them appears to have his mouth stuck open.

Is it possible that he has been fighting with one of the other red eye and has had his jaw broken?

 

[guppy]

That is very possible, if it is stuck in such a way that the fish cannot feed you could catch it and using the back edge of a butterknife gently press backwards towards the fish's tail at the hinge of the jaw, it may reseat the mandible into place if it is the hinge that is damaged. I did that with a madtom once and it worked. I figured that like with your red eye, there was nothing to lose as it would die anyway if I did nothing.