Plain emoty tank, no sand or gravel or rock. Just some pvc
pieces for the fish to hide in. A heater and some type of filtration. Lots of people use sponge filters that they keep in their up and running tanks and when they need to qt they just move the sponge filter over.
I'm lucky in the fact that I kept my 55g running with its own filtration for the last few months. Now if I had to treat anything in it I would have to move it though since I have some live rock and sand in the 55g