Originally Posted by sango
Its about 4 weeks old.
That would be one major contributor then, butterflyfish are iffy at the best of times. What could actually be the problem is the fish is not comfortable with the "growing pains" the tank is undoubtedly still going through. You may not be dealing with a parasite at all, rather unstable tank parameters.
I would suggest making sure you stay on top of the weely
water changes, keep strict tabs on the water quality and be sure you keep trying to find a food that the fish might eat. It's not going to last long at that rate. Personally I would urge you to return the fish until the tank is ready.