Sorry for the long post, but I want to make a point. A few Q's.
1) what size tank
2) how many fish
3) how old is your tank(post cycle)
4) what are your water Q readings (Am, Ni, Na
As roka says, you would have to remove the fish, treat them for SW
ich in a seperate QT tank and have them removed from your DT for at least 6 weeks post the last time your see a white spot on your fish. This will kill ich that is in your tank now. From then on you would have to QT each new addition not only for the the possiblity of ich but for any other problems, ie helping it to regain it's strength, seeing if it eats ok and calm it down from all the stress ect...
It is always best to QT only one fish at a time because "and think of this" what happens if each new fish in your QT would come down with a different diease, how would you treat that!!! Get the picture.