Freshwater dips do almost NOTHING. If they have ich there isn't a real big chance that ANY will die. It has been proven, from what i've read, that some will fall off of the fish, same with aquarium salt in freshwater tanks. The salt makes certain types of ich fall off...but barely enough to save the fish.
Most saltwater fish can't take being in freshwater for more than a couple of minutes, barely enough time for it to kill any parasites wedged in they're intestines, etc. If you think they have something try hyposalinity if you think it's a must. I would much rather someone QT and possibly use a little medication here and there than anything else though. If your fish has a major ich infestation or disease there is most likely something wrong with the tank, temp, etc. Trust me, when I started I wasn't the best "fishkeeper" and I had ich...once. Since then I have never had any of my saltwater fish get any disease or parasite...and in saltwater condtions it shouldn't be seen as much as in freshwater tanks. If your getting ich a lot that probably means your fish are stressed, temps are too low, or something else.
Ah well felt like typing lol.