I have a question and was wondering if anyone could answer it. I bought a goldfish and three fantails from a pet store about a 6 weeks ago, and until last week the goldfish was a gorgious gold color, last week I noticed that he was startng to change color, his dorsel fin changed over night to black, now he is about 50% black. I don't want it to harm the other three fantails in the tank, I am very concerned. Is the a mating right of mature fish or is it a disease that will affect the three other fantails in the tank?

In case it matters the fantails are two gold fantails ( they look black, until they move under the light in a certain way then they are extremely gold) and a calico fantail that is my favorite.

If it is a disease, what do you suggest I do?

The color change may be perfectly normal. How big are the color changing fish? I suspect they are still small (less than 4" say) in which case color changes are very common. See my sections on this at
http://fishpondinfo.com/gfish.htm#health under color changes and
http://fishpondinfo.com/gfish3.htm#change
Color changes can come on quickly as you've seen.
There really aren't any diseases of fish that would cause that kind of color change. Diseases might cause the fish to white-out or become pale or darken up but not change color totally. The black on the fins is very common during the color changing process but it will usually change back to orange or white on the fin tips for most commonly colored goldfish. A blue oranda I had years ago developed a bronze patch on him and white fins when he was full grown out of no where so color changes are to be expected.

Thanks Robin, I was really worried about it, and this has helped me out tremendously.

Thanks again...