My Q tank is a 10 gallon tank. I set it up on an as needed basis. The rest of the time it sits on the self. 10 gallons is not big, but I always buy small, juvenile specimens. Some folks will recommend larger Q tanks for larger specimens.
I have a FO tank with a wet/dry. I borrow a few bioballs and put them in an aquaclear filter when I need the Q tank.
Besides some filtration, I recommend a place for the fish to hide. I use a couple of pieces of PVC for that purpose. I put the tank in a subdued lighting area, so as to further reduce the stress on the fish.
