he's a pretty little fish.
sure looks weak though :-(
I'm not seeing fin rot in the pics..? do you see something that doesn't appear in photos, or are you assuming that missing fins rotted off?
maybe the other fish tore off fin.
comet goldfish are faster & tougher than fantails. it also could have introduced a pathogen/sickness which had started weakening the fantail before being attacked.. fish will pick on weaker ones.
I'm not sure what to tell you, as he appears to be dying but I could be wrong.
clean water is good, but it'd be nice if it was well oxygenated as well (but not a strong current which uses his remaining strength) oxygen needed, especially since he isn't swimming to move water through his gills more.
If you're lucky, he may be in temporary shock from the beating & recover strength in the safe bowl.. give dim lighting.
I'm sure others will help too.
If you really see abnormal swelling in his gut area, adding dissolved epsom salts may help. it helps reduce swelling and help move the digestive tract. is ok to use along with regular salt. they do 2 different things.