Medicating fish is an important and often controversial topic in the fishkeeping hobby and industry. It is my opinion that it is the job of fish wholesalers to do as much medicating as is required while they have the fish, so that when the fish go into the retail store they do not require any medication. Oh, if wishing made it so. The reality of handling aquarium fish is that at every level there is always the chance that fish will break down with some disease or parasite.
Every time I receive fish into a new tank, or move them from one tank to another, I always treat with a dose of QuickCure. QuickCure is formalin and malachite green, and it is the best (for me the only) prophylactic treatment that should be done for any fish at the retail level. Used at the standard dose, it will not discolor the water, but it will provide some protection against ich and fungus, the two most common maladies for aquarium fish.
When it comes to using antibiotics on fish, I do not think this is something that should be done – either at the retail store level or, most especially, by hobbyists. Unless you have a microscope and know how to use it, it is impossible to diagnose what bacteria are causing the problem on the fish (assuming it is bacteria and not just lousy water conditions, which can be remedied by a couple of partial water changes).
Because you don’t know what the bacteria are, usually stores/hobbyists resort to using a wide spectrum antibiotic, which knocks out both gram-negative and gram-positive bacteria. The problem is that it also knocks out the good bacteria of the nitrogen cycle and the tank has to rebuild the cycle. In the U.K., hobbyists cannot get antibiotics except from a vet, and here in the U.S. one of the big-box chains has stopped selling (most) antibiotics. Thinking there is a bacterial infection that requires antibiotics is very dangerous and treating with them can do more harm than good.