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Basically none that I am aware of. Depending upon your relationship with the store, you might be able to have them "quarantine" the fish for a couple weeks if you buy them. I really don't know if any store will even consider doing this, but it is a thought. Aside from that, if you acquire the fish they need to go into something so the display tank is all there is.
For 25+ years I never quarantined fish, and I only had ich once or twice, and once some wild cories came in with gill flukes. But then my luck changed...twice within two years I introduced an internal protozoan with new fish and lost almost half the tank before I could get it under control. Internal pathogens are invisible, you have no way of knowing they are present until the fish dies, and by then it has spread to most of the others. It is impossible to treat for "x" when you have no idea what "x" might be, there are many diseases. And remembering that stress is the direct cause of 95% of aquarium fish disease, preventing stress is a big game changer. And that means no chemical additives or medications in the tank unless you know the fish have "x" and it is the safest and best treatment. There is no "preventative" treatment for fish because the variety of possible diseases cannot be addressed by any general "tonic."
Next time there is a sale, pick up a 10g tank as a back-up/spare/QT. A simple sponge-type filter (I like the Fluval Quiet Flow internal filter, the smallest one would hang in the back corner and they are very good sponge-style filters. And a heater.