I have a little 3" one. I read somewhere that they change shape as they get larger. I saw a photo of a large one and it was imo ugly. The fin doesn't grow with the rest of the body as they get longer.
With these fish being so commonly available, I'm surprised there aren't more being kept by MFK'ers. Every pet store has them but I guess people don't buy them as often as I thought. I'd like one someday when I have a pond.