I don't have any pictures of the black spots, and since I don't have a digital camera, I probably won't be able to get any anytime soon. To answer your ?'s, I have a gravel substrate, I vacuum every 1 to 1.5 weeks, the tank is about 3 years old, there is no measurable or detectable ammonia using test kits, and yes, I do believe that these spots came from a host fish. THe first fish I had with these spots was around a year ago, and was a Columbian tetra. It started out as small black specks, which grew into patches, and eventually covered enough of his middle section that he died a few months after showing the intial tiny specks. And, since then, it has a been a few tetras here and there that it has appeared on--serpaes, lemon tetras, Columbian tetras, etc. And, I have a separate tank with a Red-Tailed (or pink-tailed) Chalceus in it that also appears to show some dull black tinted areas on his funs now too. A Chalceus is in the same family as tetras, and is the only fish in that tank with symptoms--there are mostly cichlids and larger, more aggresive fish in there. So, since he is related to tetras, and they are really the only ones with symptoms in the other tank, I think they must be the most suseptible. I wasn't too concerned with a couple tetras here and there with it, but I didn't want my discus, angels, or cichlids to get it. But, now when it appeared in my other tank (and I try to be very careful about cross contamination between tanks), I am concerned. That Red-tailed Chalceus isn't easy to catch and gets spooked very easily, so he would have to be treated with something in the original tank, which is a 90 gallon. That could be $$$$ for meds. But, the tetras are in a 30 gallon which I don't have any problem treating the whole tank there if I have too.
I am suspecting that it is myxobacteriosis, and I might try treating for that with a combo scheme. I will do more research on that and see what I can come up with. One person had luck with that Paragon II, but it isn't available anymore. I really wish Aquatronics was still in business, because in my opinion, they were the only company that made any meds that REALLY worked! I had really good luck with their stuff in the past, so I will have to see what I can come up with from other makers. Please post any other ideas or info if anyone has any. Thanks.