Sick black tetra

Question
QUESTION: We have an established tank with a danio, an algae eater, and several black tetras.  This morning we noticed that one of the tetra's side is almost completely black.  There seem to be no bumps or sores, and no fungus that is visible.  It just kind of hangs out in the back of the tank near the surface.  I have searched my fish book and have found no description of this condidtion.  Do you know what might be wrong and how to treat it?

ANSWER: Hi Michele,
  Is it a very distinct portion of the fish, e.g., the entire one side, but not the other, with very distinct edges as to what is black and what is not, almost like someone painted solid black paint on a portion of the fish?

-- Ron
  rcoleman@cichlidresearch.com
  Cichlid Research Home Page <http://cichlidresearch.com>

---------- FOLLOW-UP ----------

QUESTION: I just checked out the fish, which is struggling to stay afloat. The black "splotch" is on one side only, roughly between the 2 normal vertical stripes. It is irregular in shape, has pretty distinct boundaries, but does not strike me as looking "painted." Also, on both sides of the fish the dorsal and anal (?) fins are much darker than normal, but not the tail.  It is almost black from the 2nd vertical stripe to the back of the fish. We used to have 4 black tetras, and another one died from this same thing.  The remaining 2 look perfectly normal.

Answer
Hi Michelle,
 Hmmm.. I was thinking it might be nerve damage but that would cause a whole section of the fish to go black, e.g., the front right side.  
The fact that it is not on both sides tells me that it isn't a "normal" color change, such as when the fish is thinking about breeding.

 To be honest, I don't have a good answer.  Sorry.

-- Ron
  rcoleman@cichlidresearch.com
  Cichlid Research Home Page <http://cichlidresearch.com>