You're best bet it to try and find new homes for the aggressive fish. If they are aggressive, then that usually only gets worse not better. It will be a problem as you try to introduce new fish to the tank. LFS likely will not take the tang due to his condition; and, as you have guessed, if they did take him, his outlook would be poor in his weaken state in a tank with other fish in the LFS.
My opinion is that the primary problem is the aggression, followed by stress resulting from the aggression as well as the tank change-over. Just for info at this point, when moving fish as you did, it is best they are keep singularly, not together until the transfer is made. Also, when you add new fish, ask the LFS to separate fish into their own transport bag, rather then putting them together. This fish was experiencing aggression in the tank already prior to the move, as you have noted the nipped fins.
Removing the aggression should go a long way to resolving the issue for this tang as well as other fish in the tank. Now, the fish seems to have also developed a bacterial infection in the eye; this too would be a result of the stress it has experienced. Hopefully, this will clear up if you remove the aggressive fish, and start improving the water quality of your system.
Are you new to marine aquaria?
Quote:
I talked with a guy who says to introduce bacteria to try and lower the nitrates
Nope. Doesn't know what he's talking about. Just wants to sell you stuff.
Try doing 3 gal water changes daily for now. I would not suggest doing a major change in the tank right now with a stressed sick fish. What is your procedure for mixing salt water? Water source?