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The tang is breathing fast and has some skin discoloration. Three common causes for this are: Amyloodinium, flukes or water quality problems. Lack of oxygen can also cause rapid breathing, but generally does not cause skin discoloration.
I think we need more history about the fish and your tank. The link below my name here lists the kind of additional information that is helpful for us to know.
To start with, if you tank doesn’t have active aeration, something breaking the surface of the water, you should add that now.
How long have you had the Meyer butterflyfish? Is it eating for you?
Jay