Hmmmm, to bad you can't set something up semi-temporary for the 20gal. 6" tank depth isn't a whole lot and I'd be a bit worried about keeping fish you plan for a 90 gal in there for 8-12 weeks, possibly more if you end up treating it for something. It would kind of be like sticking someone in a closet for 2-3 months, especially if it's a larger fish that would otherwise be ok for your 90 gal.
Any way you can set up your 20 on a rolling cabinet (with good castors) or coffee table? Maybe you'll even get the GF hooked and end up having in permanent
