You can eliminate Ich from a fishless tank in one of three ways. Leave it fallow for 6 weeks, use hyposalinity on it (you will lose any inverts), or use copper (you will lose inverts and your nitrogen cycle and will not be able to keep inverts in the tank thereafter).
I would never suggest using copper in a DT reef tank.
If the DT is fishless. Leave it fallow for 6 weeks, then QT everything new that has a wet surface before placing it in your DT (fish, inverts, rock)