Most fish can survive copper treatment. However, copper can cause problems for sensitive fish like angels, butterflies and tangs. This is the primary reason that hyposalinity is the preferred treatment for ich. Hyposalinity actually decreases stress for fish while copper can increase stress. Is this really what you want to do for a fish which is already ill?
If you are using already copper, you really should use a test kit to measure the levels of the form of copper that you are dosing to decrease the possiblity of harm and to ensure treatment success.