Same thing going on over here.
I had a fish with ich in my DT and of course, I didn't have a QT setup. I just went out and got another small tank (or you could use the 20 gal
you have) and moved all my fish into it since they've all been exposed. It is risky, because it's un-cycled, but I really had no other choice.
Just make sure to check the ammonia and pH at least once every day. It may take some daily water changes as well for about a week before it levels out. I also used Stability to help add good bacteria and AmQuel to help control the ammonia if it suddenly spiked, but mainly just a lot of water changes.
Leave your DT alone and let it run empty (no fish) for 8 weeks. Inverts will be fine.