My honey gourami is turning black (see pic). It seems to be fine otherwise (eating, activity, etc...). I noticed about a week ago that the edges of the back and bottom fins were darker but thought maybe that was just something that develops as they grow. I didn't notice it getting any worse until today. This doesn't seem normal anymore. There is one other honey gourami in the tank and it seems fine. I don't know if they are male or female. What is going on and how do I fix it?