I just came home from 4th of July festivities and went to feed my tanks.. And noticed a huge chunk missing out of the middle/back end body of one of my neon tetras. Part of his scales actually bunch up behind it. To me it looks like a pretty serious injury, but I have NO IDEA how it could have happened. In the past week I moved all my other fish to a larger tank and the neons are the single only fish in the (39 gallon) tank right now, other than a handful of snails. He has lost a great deal of color.. His red under belly is really pale. He seems to be able to swim alright but he kind of sits dormant, he's not moving around much. He looks like he's in pain.. The only abrasive object I have in the tank is a large lava rock, but I'm having a hard time understanding how he'd injure that particular part of his body on it. Should I wait a few days and see how he does? Should I euthanize him? I've only ever had to euthanize a fish one time (using the clove oil method) and I'm still traumatized.. I've attached a picture although it's almost impossible to really see the severity of the wound. They move around so quickly and any time I come up to the tank they get spooked and pal up.. Any ideas how I can help this little guy?