It is really going to depend on the fish your are going to put in your QT. I seriously doubt a tang would make it 6 weeks in either of those tanks. Bare bottom with a filter, heater and maybe some pipe or rock so they can hide. For smaller fish a 20 would be fine for larger fish maybe a 40 or larger for things like tangs even small ones have need for zoom room.
Stress is the main killer of tangs, tangs even small ones need room to move. I have seen hippos that were in a small tank and in just a matter of days it had stress related illness, lateral line disease, Ich is another thing that seems to be brought on by stress in tangs more so than other fish. The less stress the better. Jaw fistsh have special needs when in QT they need to have sand to make a home in.
Another thought to keep in mind is if anything is wrong with the fish while they are in qt you want a tank small enough to medicate properly. I don't know what I would have done if any of the fish i qt'd in my 55g needed either medicine or hypo salinity treatment. Alot harder in a larger tank.
__________________ 180g Reef - 60g Rimless Reef -90G FOWLR- 125g Malawi Cichlids- 40bREEF- 34g Fresh Planted-working on- 20L FOWLR- working on
I have a mix of many different saltwater fish amongst my tanks, but I love my Tangs most of all.