For future reference, I have had very good results treating every new fish with melafix for the first 3 days, in quarantine.
Unfortunately, most of the damage done to the fish is done before we ever see them. All the ammonia in their system, in the form of harmless ammonium due to the PH of the water in the bag, becomes deadly ammonia as soon as the dealer start acclimating the fish. In the limited testing I have been able to do, a dose of ammo-lock or amquel in to the bag upon arrival has made a dramatic difference in survival rates. You do have to leave them in their bag a while longer, I don't know how long it takes to cycle through their system. I have only been able to attempt this with 2 shipments of fish.
I have also heard of using CO2 to drop the PH in a holding tank down to the level of the fish in the bags, and acclimating them to that. Then slowly(very slowly) raising the PH until it reaches normal levels. I don't have the equipment or the need to try this, but I figured its good information to pass along.
Jason