Treating fish with antibiotics

I realize you are supposed to treat fish with antibiotics in a quarantine aquarium but doesn't that also kill the denitrifying bacteria as well?  So how are you supposed to keep them alive?  Besides which, won't the fish simply be re-exposed to the disease bacteria as soon as they are returned to the display tank?  Seems self-defeating.


The reason I ask is because I just lost my yellow-purple Coris Wrasse to what I suspect is fin rot.  I started treating my DT with Melafix and Pimafix yesterday at half strength (I should have done some research sooner, started sooner).  The corals weren't exactly happy about it but recovered by the end of the day and barely flinched with the dose today.  I suspect my firefish may have the disease too so am continuing to treat to see if they improve.  In any event, it would be impossible to catch them to remove to a quarantine tank.