Help! I bought three fantail goldfish a month ago and they started turning black. First it was their tails, then around their heads. The guy at the pet store said it sounded like finrot and sold me some antibiotics to add to the water. It did not work. The fish are almost entirely black! What disease is this and what can I do?
There is nothing you can do. The fish are not sick; they are just changing color. There is a form of finrot that blackens the finnage of fish — this is the dead tissue. But if the problem really was bacterial infection your fish would be long dead.
The guy at the pet store made his best guess based on what you told him. Chances are had you brought the fish in he would have recognized the simple color change.
Color change is quite common in goldfish and koi. I just had what was a beautiful showa (a black koi with broad red and white markings) transform herself into a beautiful black and white utsuri (a black koi with broad white bands). After 10 years of being a showa her reds just disappeared, to be replaced by pearly white. Pretty remarkable.
Such changes may be caused by environmental factors (e.g., changes in water quality, sunlight, etc.), age or fish food. Enjoy your black goldfish. Keep them in bright light and they will develop a nice rich velvety black color.