Blimey, how did that happen?. He must have caught himself on something, or there is a bully in the tank. If the other fish are healthy, please don't treat the whole tank mate, get yourself a Qtank. It is in a fish's nature to pick on another fish that is less than perfect, and if you leave him in there much longer, he may not make it, either through bullying or stress.Have you got another small tank and a spare heater? if you have, move him and treat with a small amount of melafix just to prevent infection, but there is no need to add any chemicals at all. Fish tear their fins all the time. They grow back very quickly if the water quality if good.
Emma