when you say there is no ich in there, do you mean you can't see any spots on them? there are other parasites that could cause scraping. can you put them in a hospital tank and treat? I doubt if dips will give you anything more than temporary relief. how big are the fish? they may be in distress because of parasites on the gills, which could be early stages of ich or oodinium.
you might want to examine them closely --
skin:
http://www.fishyfarmacy.com/fish_dis...disorders.html
gills:
http://www.fishyfarmacy.com/fish_dis...disorders.html