Good idea setting on up.
I use a 10 gal with simple hang on filter. Good for all but the biggest of fish.
If it's for a lot of fish you should prolly go bit bigger
A 20g long or a 30g is more than enough to QT tangs or foxface. I used to have a regular 20g as a QT and upgraded to a 30g about a year ago. I always have fish in the QT.