A weakened fish will frequently be the target of other fish, so that likely is why the fish are now "attacking" him. He should be removed from other fish, or at least be in a tank where he has a good hiding spot and will not be targeted, until he fully recovers. He needs a chunk of wood with a tunnel or crevices so he can escape and hide completely. An artificial hiding place can be used, such as a half flower pot (made of only clay or some safe material, no plastics etc.) or a rock cave. Somewhere to fully hide.
I won't comment on treatments; when we have photos others will be able to advice on this. Frayed fins can be due to several things, nipping obviously, but also water conditions, the stress of being out of the tank, etc.
You answered the question on parameters as "fine," but that tells us nothing. Please give us numbers for any tests. We cannot adequately help if we don't know all the issues/data.
Black Skirt Tetras are notorious for nipping fins. You may not have seen this, fish often can be sneaky and not misbehave when you are present as you distract them and they associate you with food, etc. But it could occur during other times. Having a larger group can sometimes help, but this species is a fin nipper of slow or sedate fish, like plecos, so this is probably something you should look at resolving. Don't keep them in the same tank.