That's rather curious. Be sure to fill us in if you're able to get a diagnosis.

Have you tested the water? If the water quality is good then it may an infection of some sort and will need to be treated with antibiotics. That would explain why melafix isn't working. First - check your water.

I would start an anit-biotic as soon as possible, you can do a couple at a time, like some that the fish will eat, and some that you just place in the water. I like to use a broad spectrum anti-biotic, but becareful, because your fish can become immune to them. I would also use some aquarium salt as well, for the stress.

try putting up the temp a little higher and some aquarium salt. If possible try to put the fish into a tank by it's self, so that the other fish wont pick on the wound, while it's trying to heal.
Could it be mouth rot?
Hope the fish recovers!!

I had a fish that i got a lfs, and it did the same thing. I just put him in a sick tank, and added some anit-biotics, and some salt, i left the temp. at 80 degrees. It took about a week to him better, but all went well. I used tetracycline and penicillin, both together, and it did the trick along with the salt.