Honestly I doubt it matters. Think of the dilution factors involved. Even if there is 100 microliters of water/slime coat on the surface of a fish with therapeutic levels of copper, consider the following: if you move the fish to a 10 gallon tank, the tank contains 3,785,000 microliters of water. So your 100 microliters of medicated water/slime is diluted 1 ul of medicated water per 37,850 ul of tank water.
It's super super dilute.
If it would make you feel better you can give him a quick bath in a cup of tank water first, but it seems overkill to me.