There is a reason why the butterfly has come down with ich, some other inhabitant, or something in the tank is stressing the fish out. I assume you have a quarantine tank right? If not, shame on you, get another tank up and running ( a twenty gallon would be ok) move the butterfly to that and go from there. If you don't have the option of a quarantine tank, well then you can try and medicate the butterfly in the tank that it is in, but something in the tank is stressing it out, you can start to see the problem that the absence of a qaurantine tank creates. If no one in the tank is picking on it, and EVERYTHING is as it should be (no temp fluctuations over 2 degrees, no ph fluctuations of more than 1... etc.) then there is an outside possibility that the butterfly will benefit form medicating even within the show tank with the other fish. Is the butterfly a new arrival?