If you see ich on any of the fish then theres ich in the whole tank and its there to stay until you do a fallow period on the tank. Taking the fish that show ich out isn't going to fix anything.
If anything, ich is a sign of stress as a healthy tank can keep it at levels where its not problematic. The stress is likely due to the overstocking.
Mebbid's Fishtopia: Various builds
"The simplest explanation for some phenomenon is more likely to be accurate than more complicated explanations." -Occam's razor