Help! Injured or Sick Diamond Tetra

Just looking at my fish and noticed something weird on one of my Diamond Tetras. It is on both sides of his body at the end of the body just before the tail. It looks almost like a bite was taken out of him on either side, but I can't tell if it is injury or sickness. There is nothing else like this on any of my other Diamonds or any of my other fish. The diamonds were the first fish in the tank and they were added about 5 weeks ago.

Should I move the affected Diamond to my spare 10g? Or is this something that good water quality will eventually heal? He is acting normally otherwise.

Sorry the pics aren't that great. I'll see if I can get better ones later.

Thanks in advance for the help.

Not sure on paramaters, but will check and post later.

No aggression that I have seen. When I first got the Diamonds there was a but of chasing amongst themselves to establish a pecking order I assume, but I haven't really seen that behavior lately. And my Angel pretty much ignores them.

Any idea what it might be from looking at the pics?

Ammo is 0, trites are 0 and trates are 10. No one else appears to be affected, but I have moved the little guy to a 10g hospital tank with the extra aquaclear 30 that had been running on my 20g. What should I treat with? I can go to my LFS tomorrow and pick something up. In the meantime I'll monitor his water quality and make sure he doesn't get any worse. I can tell that being away from his buds is already making him a little stressed out, though.

He is still acting normally today, and looks about the same as yesterday. I can go to my LFS to get medication today, but I'm not sure what to get. Also, I can't figure out what this is. I can't seem to find any type of disease or infection that affects only this particular area on the fish.