Why does my fish not eat and has a disintergrated tail?

Hi, I actually have a no tailed goldfish, he was like this when I got him, he has a stump in place of his tail but he gets on fine with my other goldfish. I'd recommend adding aquarium salt in his quarantine tank to help healing. if youre lucky he may turn out like wiggle, my goldfish, deformed but alive and healthy. Good luck