Its a 20 gal tank. Ideally you only QT 1 fish at a time, unless you have a school or mated pair. I plan to have smaller fish in my tank, so the smaller QT size is fine for me. If you are going to add larger fish, then go with a 30 gal.
When fish are in the tank, I just do a gal of water change a day, with water from my main tank. If I do hypo, there is so many water changes involved with that, that I don't have to do additional changes except during the 3 wks of hypo itself.
There is a lot of circulation in that little tank [3 outlets]. Haven't had a problem.