Its not that the fish you bought were more suseptable to ich. It's that all marine life are not suseptabe<sp to bad water conditions, stray voltage, and poor diets. IMO You didn't pick the best beginer fish. I feel bad for your luck, but the only way to be lucky in this hobby is to learn, listen,and educate yourself. How fast did you drop the salinity?
How long has this tank been set up?Why don't you just bring the fish to a lfs, and trade them in for store credit?before they die! what was causing the voltage a ph? heater?