PLEASE HELP MY 4 INCH GOLDFISH?

I have a goldfish in a 15 gallon tank with 1 guppy 1 puffer fish 2 fish that are small and kinda chubby that are black ad white and 3 tiny little yellow ones that just eat off of what ever touches the bottom. My gold fish is 4 inches long and is suddenly missing a lot of scales and is bleeding around his fins tales and tummy area!! I heard puffer fishes can hurt gold fishes but the start by chewing off the tale. His tale isn't chewed off. But now the tiny other fishes are eating off him :( I know this is stupid bc it's a fish but he's my only pet and has lasted 4 years I got him at a fair and wasn't even an inch when I got him. I separated him so now he's alone in a tank and I have been feeding all of the fish.... Oh and this started when I got the puffer fish and the 2 chubby white and black ones!!!! Please please please help what do I do is there something I can do?