i think my fish has a disease

Question
1 of my goldfish has like a black spot on the end of his tail and im very concerned about it he didnt have that spot when i bought him so i realy dont know please help

Answer
Just one black spot? I would not worry about this too much. Fish to change colors sometimes. It usually has to do with maturity or your water quality is different from the pet store's. I know my fish was pale in the pet store tank then turned so colorful. Don't worry about it unless he shows signs of being sick.