A 30 gal
would work very well. You will need a heater, powerhead and/or HOB
elbows and the like for hiding and something to prevent jumping. Eggcrate works well. A light is not required or recommended.
Since you will be/may need to treat with an antibiotic, cycling will be unimportant as the med will destroy the biofilter. Fill the QT
with the water from the main to match parameters as close as possible and not stress the fish further. Corral with a plastic container instead of a net if at all possible. You will need to monitor water conditions for NH3
and such so be prepared with extra SW
for water changes. As much as 2x daily may be needed. Don't forget to add the appropriate amount of med for the water that's removed at each water change.