I'm worried it's Viral Hemorrhagic Septicemia
More likely bacterial IMO. We have had a few fish wind up looking about like that and it was always IMO from fighting. We would almost never actually see it happen, indeed in a few cases we went to bed thinking things were fine and woke up to find a fish barely able to swim, missing most of its fins and/or lying on the bottom of the tank looking nearly dead.
We have revived a couple form this state and have lost a couple. We are nursing one back to health atm and our plan for when this happens is now something like this:
Immediately remove such fish to a hospital tank, keep it reasonably warm (we usually go for 82F or so) and well oxygenated. Cover the tank up so it is dark so the fish feels safe and doesn't get agitated.
Treat with antibiotics, we use Erythromyacin. We think the fish we lose we lose to secondary infections, we often see septicemia type blood spots through the scales, the fish are lethargic and they universally lose most of their tail fin. The rest of the fins also tend to look ragged.
Sometimes we combine this with Pimafix and Melafix to try and stop external fungus and infections.
If one of my fish looked like your picture that is what I would do. I would also be very alert for signs of bloat - notably fish not eating or spitting out feed - and be very quick to Metro the tank (which has virtually no downsides if it is not needed).