Actual fresh water. The fish themselves like to take freshwater baths in the wild whenever they find a supply of fresh water. It hydrates them and makes their parasites either burst or at least swell up so much that they can't stay attached.
2-3 minutes is plenty, actually close to the safe limit for many fish.
NOTE: do not use water which has been dechlorinated with chemicals. These chemicals cause very rapid slime production in the fish, and this slime defeats the purpose of the dip by shielding the skin & the parasites from the fresh water.
Split open bivalves sometimes work, sure, and they're certainly worth a try, but I've personally had much better luck with a glob of ground up blackworms & anemones, preferably with some sort of frozen food ( fully thawed ). Such a presentation evokes a feeding frenzy among the fishes in the tank, and the butterflies eventually join in the fun. I should mention that this only goes for Atlantic butterflies, and that those obligate corallivores of the pacific won't touch this stuff.
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