I have a fish like that too. Started when he was a tike a few years ago. His tail has partially grown back but the side fins and top fin(s) are gone. No other fish has ever shown any ill effects. No idea why.
I do know that he eats like a horse. Steals most of the sinking pellets before they can even hit the ground for the bottom feeders. He swims basically fine though sometimes looks a tad odd.
Other than that he has seems happy, fat and fun loving for the years of his life. I worried at first then just paid it no attention at all. My little 'special' fish. *sniff sniff* You'll never know the joy a fish with a special gift can bring you until you have one *sniff sniff*
Other than that, nope.. no idea