I am not sure of the right things to call the different fins, but I looked up a picture and I believe it is one of his ventral fins. This issue is about the same poor boy who injured his head a couple of weeks back.
At the end of one of them, it looks odd. There's nothing white or black, and he doesn't show any other signs of an issue, but it still worries me. It's difficult to explain, but it sort of looks like his fin is turning clear. There's red in the middle of it, like it's his fin, but around it has became clear?? Can anyone help, please? My camera can't get a clear picture. :/