Thats something I have recently been dealing with myself, I have a fancy guppy fry that is litteraly and "L" shape. The rest of the bunch is normal, it just looks like he is bent in half backwards, but so far he is doing fine and dosn't seem to know that he is strange. So far as growth is conserned he is keeping up with his siblings. He looks so happy and is doing so well ( and im a big softy ) i could never bring myself to do him any harm. I figure i will either start a smaller tank just for him to make sure he dosn't breed or give him to one of my friends that couldn't breed a fish if her life depended on it.