fins will grow back, just like your nails and hair. Back when I was keeping african cichlids, what I use to do was to chop off the tail fin of the dominate male, so that the females and subordinate males would be able to get away. By the time his fin grew back, he wasn't chasing the other fish anymore since he thought they were much faster then he was. Your fish needs more food/calories. He only has enough to support the current length. (It's possible there are bacterial infections along the fins edges.)
Going back even further, when I kept goldfish together with my little red ear turtle, he would routinely take neat triangular bites out of the goldfish tails. Of course if I had only 1 goldfish, eventually the turtle would get him, but I kept 10 or so in there, so all my goldfish has varying tail shapes as they grew back.