HELP!!!!

Question
My black molly has a small brown spot on his side and is being very aggresive to a sword tail and a Mickey mouse platy.
In the tank, most of the time he's very calm, he's never
been aggresive to a fish. The thing that worries me is that he is very big for a molly and could easily hurt them. The spot is right behind his side fin I don't know if he hurt his self or is sick?  

Answer
Hi Katie,
The brown spot sounds like natural coloration to me. If it doesn't form a bump or any odd appearance to it then I guess it'd be just a little extra spot of color.

As for the aggression. That is common with livebearers. Likely he is a male being territorial/dominant aggressive towards them. There's not much you can do but try to provide some visual barriers with plants and retreats for the fish. But even then, it may not work. You may have to separate the molly or may have to take him to a petstore if he is making the lives of the other fish miserable.

I hope this helps and best of luck!
Karen~