What is this brown spot on my black phantom tetra?

A couple weeks ago I noticed a white patch before the tail fin on one of my black phantom tetras. I thought it might be columnaris so I promptly put him in quarantine and began treating with Furan 2 and some aquarium salt. I went through two rounds of treatment, and it has since turned into a brown patch instead of white, which made me think it might not have been columnaris after all. Is it something else? I dunno. Is this normal? Velvet maybe? I don't see any signs of it anywhere else on his body and he is eating normally, swimming fine, and doesn't show any behaviors that makes me think he's sick.

None of the other fish from the main tank have shown any signs of disease.

There also seems to be some black spots in the brown patch, but I think that is the tetra's normal black coloring trying to come through.

Ammonia: 0ppm
pH: 7.4
Nitrite: 0
Quarantine tank size: 10 gallons

Will you please circle what you are talking about? Are you speaking of the spot near the end of his tail? (On his actual body?)

Here's another picture from the other side.

A little update in case someone else has any thoughts. The tetra is still alive in quarantine, acting ok, but the spots have not disappeared and have in fact progressed into large lumps on both side of the body. Other than that he seems to be ok. Any ideas? Tumor? Should I put him back in the main tank? I'm worried that, although it obviously hasn't killed him, that it might spread to the other tetras.