I like to have a stocked medicine cabinet.
My preferred antibiotics are kanamycin and metronidazole, followed by tetracycline as a distant second choice (it's less effective in hard water). I typically don't do bath treatments tho unless the fish aren't eating as I prefer to make up a medicated gel foodhttp://thegab.org/Articles/MedicatedGelFood.html
for a bath treatment, maracyn plus is good as is kanamycin and metronidazole (depending on symptoms).
For external parasites, I keep formalin and quick cure on hand and praziquantel for flukes.
I keep salt on hand, but only recommend it for specific things (e.g. preventing nitrIte poisoining or treating ich since ich is likely not salt resistant), however, some external parasites will not respond to salt these days. Noga identified several strains of costia that can tolerate salt.
Potassium permanganate for disinfecting things or as a shotgun treatment with the biofilter pulled if I'm not sure what's going on just to knock whatever back while I figure out what the problem is.
finquel (MS222) for sedating or euthanizing fish.
Most times, I do quarantine new fish and I always disinfect new plants.
Do you guys ever use a microscope to identify what's bugging your fish?