Problems with my Tetras.

[ChileRelleno]

Some of my Lemon and Neon tetras from my 75gal community tank have developed a dark colored thickening/lump at the base of their tails. They are all eating and behaving as normal, with no signs of distress. I could not find anything similar in description to this in books or online, so I took'em in to my local Fishstore and at first glance they said "parasites" but then got to looking closer and thought perhaps damage from a tank bully (they all seem to get along fine) and then looking even closer they said "Hmmm...? Thats really strange?, we're not sure :? ."
They suggested treating as bacterial infection.
Has anyone here seen anything similar to this on tetras or other fish?

 

[Allivymar]

Not I Chile; can you get us a picture? Maybe we can figure things out when we see it.

In the meantime, can you give a more detailed description? Are the fish with the lumps acting any differently, maybe breathing harder then the rest? The lumps, are they smooth? Erm...lumpy? Is the tail itself changing color? How long have they been in the tank/how long has the tank been set up/has there been any new additions in the past month? How long have they had the lumps? Do they appear overnight or grow slowly? Exactly where are they located; right at the base of the tail itself or just fore or aft? And how many are affected?

 

[ChileRelleno]

Allivymar, thanks for the response.
I'm sorry to say that I cannot provide pictures due to the fact that fear of some sort of tetra disease influenced our decision to euthanise these particular fish .
Let me see if I can provide a better description of the problem by answering your questions, and perhaps get a better idea for any future occurence.
The tank is well established with 0 ppm A, 0 ppm Nitri and all other water qaulities in balance. There have not been any new additions within the last month. The fish with the "lumps" had been in this tank for aprox 6 months. The lumps appeared slowly over about 2-3 weeks and started as a discoloration at the base of the tail. The lumps were right at the base of the tail and did not involve the tailfin, the lumps were smooth and involved both sides of the body. The darkening of the color was very dark in the affected area and did not affect the tailfin. The fish affected did not under observation appear to be behaving differently or exhibit fast or labored breathing. There were three out of nine Lemons and one out of eleven Neons affected.
I closely examined the fish by hand after returning from consulting the LFS, the lumps appeared to be within the flesh (no ulceration) and didn't IMO look like tumors and didn't look like any parasite that I or the LFS know of. Just a dark (almost black) thickening of the flesh at the base of the tail (on the Neon the red stripe showed thru but much darker).
The Lemons are normally very fiesty amongst themselves, I haven't seen what I would call overt bullying just a well established pecking order. Perhaps I should get rid of the Alpha fish