You can also try rearranging your rockwork to remove the clown's comfort zone. (Do this while you are acclimating the new guy.) Be sure there are places for both fish to hide for a while. Leave the lights off when you add the new one in, overnight is fine. When they come back out, the clown will feel new, also. No guarantee, but it has helped me in the past.
Otherwise, you may have to wait until you can upgrade. A 20 doesn't give a lot of room. The clown can protect his sleeping spot and still "police" the entire 20 gal. He can't do that in a 55 (for instance) without leaving his sleeping spot unguarded.