Melafix has astringent properties that probably irritated your snails' exposed mucus membranes.
I've read that some catfish, bettas, and gouramis show the same 'dazed' reaction to Melafix. The med most likely irritates the scaleless skin of the catfish and the lining of the labrynth organ in the betta and gourami.
In general, snails and other inverts seem to be extremely sensitive to most meds. It's always best to move them to another container while medicating fish. Before returning the snails to a medicated tank, do a BIG water change and run carbon in the filter for a day or so to remove the meds.