The bottom right fin is turning black with my goldfish ranchu. I tested the tank, and the perimeters look good to me and all the other fish in the tank or thriving. He started last night staying in the bottom left side of the tank and comes in and out. He stays Most of the time by staying away from the majority of the fish. I don't see any aggression in my goldfish tank. I was wondering if y the Blackfinn causing it. Any treatment recommendations