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I don't keep any on hands. If a fish gets sick, put them in QT, and monitor them closely for a couple of days. Diagnose the issue, then see if a medication or letting it run it's course is better.
I'd recommend learning what your lfs carries and how late they are open. Over here anyway the stocking is terrible. Also see what is online so you know what options there are.
If you can't get to the shop same day I'll dig up some options.