I really need some help. One of my fantail gold fish is really sick. I've had him for almost a year. I'm not sure what it is. I noticed a black mark like a streak on his side about a week ago. I used some melafix for a few days, but now it looks like it has bruises on the underside of his belly and the black streak mark is starting on the other side. He didn't seem to be sick, but now he is not swimming so well. I have checked the amnonia, nitrites and ph of the water. All levels are good. Please help. Thanks in advance.