Columbian tetras are sick - diagnosis?

Hello all. Two of my seven Columbian tetras appear to be fighting for life.
The thing is, I've never seen a fish hang on for this long (4-5 days now). Usually, if they seem sick or dying, they're dead within a day or a few hours.

Symptom: skin has patchy black markings, as opposed to the healthy ones, who have a clean, shiny silverish-blue color. The gills also appear inflamed. I can actually see the gill filaments as the gill plates have either shrunk or are constantly flexed outwards. The fish will hover down near the bottom hovering kind of sideways (like they're dying), but then later I'll see them swimming with the other fish (though not as lively).

I could try taking a picture tonight, but I worry that my camera will not capture a good, clear shot of this.


I did a search on google for fish diseases and found this website:

http://www.fishyfarmacy.com/Q&A/changes_in_color.html

I read this symptom, which seems similar, but it doesn't address the gill problem and I don't want to euthanize if I'm not 100% certain.

Q: One part of my fish has turned black and there is swelling underneath the area.
A: Sounds like a Melanocarcinoma (tumor), that has pinched off the color controlling nerves of the fish

Treatment: No suitable treatment known. Sacrifice this fish humanely.

Click to expand...

Any ideas? I don't mind euthanizing (if I can catch them), but I'm mainly concerned about whether it's treatable or whether this can be contageous to other fish. All of the other fish and shrimp in the tank appear perfectly healthy.

-Ryan