I done it in the past for butterflys, tangs, triggers and large angels. Usually they all handle it well so I leave them be for 10+ mins. Usually you can tell if a fish is taking it well or not. Those that do seem to act normal like they are in saltwater. Those that don't will start lying on the side or start gasping at the top.
I have a wait and see approach for smaller fishes. Usually I'll see the twitching or suspect flukes when buying the fish, I dip. If i put them in already and they start twitching later - then I dose PraziPro.