For a hospital tank 1 gallon sounds good. But, for a QT, I think a 5.5 gallon minimum. Personally, I believe a 10 gallon would be best, if you have the room.
I'm QTing 4 " emerald green cory" and my 5.5 is just too small. I upped the filtration to 2 aqueon quite flow 10, 1 aqueon quiet flow 20, and a Caf-10 (sponge biomedia). All have seasoned biomedia. The problem is the emeralds are huge. Close to 2" each.
I started them in an emergency 3 gallon doing 3 PWC
a day. Shortened the QT on my fish in the 5.5. Then, moved them and kept adding filters with used biomedia until all tests 0 for 24hrs. They are cramped in the 5.5 gallon, but I have no room to add a 10 gallon QT anywhere.
The problem is 5.5 gallons is just too small of a quantity of water when wastes are made.
(Friend of Bill W., One day at a time)