I wouldn't order medicated fish food.
Fish die after arriving in the store for a lot of reasons. They've also often been heavily medicated already. It's not necessarily the case that the dead fish had a contagious disease.
And even if you do bring home a bug, it's hard to say which medicine is the right one.
Finally, as in humans, some of the things that cause fish disease are present all the time. They cause illness only when things get out of balance. This is one of many reasons why overuse of antibiotics is such a bad idea.
For instance, did you know that one of the bacteria that causes pneumonia, sinus infections, and ear infections is always living in your sinuses? You only get sick when, in the community of bacteria and fungi and viruses that are always in your body, that one bacteria population gets out of control (often because something killed the other stuff off).
Your best bet is to provide a very healthy home. Do avoid buying when it looks like the dead fish was diseased, and quarantine as is appropriate to your situation (I don't, but I have a small tank of cheap hardy fish). Medicate as rarely and judiciously as possible.
Every time I have bought fish, it was from a store that had a few dead fish somewhere. The long term results from PetSmart and from my LFS have been the same.
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