ive kept a powder blue and naso, and never battled with Ich (fingers crossed). IMO I believe it has a lot to do with the food soaks I use (selcon and h20 life supplements). My orange shoulder sprouted black ich about a week after I got it and i didnt do anything to treat it except a good diet and it got over it fairly quickly and nothing else caught it.
Although its not a guarantee, i believe a good diet+food soak helps a lot