large white spot near base of tail

[tanana]

My six year old albino rainbow shark (only fish in 25 gallon tank) has one large white pimple-like spot near the base of the tail. I've treated it before with ich meds and bacterial meds and it started to get smaller. Now, after several weeks, I can see the spot getter larger again. It is not cottony and I don't see it moving as you would a worm. The fish is not in distress. What can that be?

[Glasstapper]

Is it like a lump under his skin? Recently I had a gourami that started out with a white lump under his scales then it slowly grew larger and turned red and ended up being an ulcer. My gourami also acted normally and was still eating and swimming around. It was bacteria, though.

OR maybe with your shark, it is a parasite like a flesh worm or something under there. If you've already treated the possibility of bacteria, then maybe you can try treating for parasites.

It's a thought.

[tanana]

No, it appears to be on top of the skin, at the very base of the tail before the tail fin starts. It it raised like a whitehead pimple, with no other discoloration.