I meant making a small chamber in my sump completely sealed off from the rest of the system and throw a spare HOB on itAn isolation chamber in the sump doesn’t accomplish what QT is best for. The two main points are to avoid introducing parasites like ich and velvet and being able to treat with medications that will or can harm the inverts and corals such as copper. Neither of these can be done in the sump.
That being said, unless you are buying large fish, a 10 gallon with a hang on back filter and a heater is all you need with some biological media that you can seed in your main tank, and use to establish your qt. Doesn’t have to run all the time. Set it up when you plan to get a fish, or can be set up when you get home with a spur of the moment purchase if you plan ahead.