Is the area raised? It could be a wound that got infected. Is the whole thing bumpy or is it just a single area? I'm leaning more along the lines of an infection of sorts at this point.
1. Gotta get it eating, what have you tried at this point?
2. Need to figure out if its a fungal, or other type of infection
3. Treat the fish
If you can get closer photos of the injury we might be able to tell you what kind of infection it resembles. You also may want to try sending some photos to wet web media, or starting a thread on RC for more feedback.
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