Hi Ruby,
The Q-tank question is definately a sticky one for beginners. If you've barely found the room for your first tank, barely talked the other family members into accepting all the new hobby stuff in the house, it can be a stretch to have a full-blown Q-tank in addition. For raw beginners, especially ones with inexpensive community tank stocking, you may want to just risk it on the first few additions, always re-asking the question: "Is my tank so good now? Have I become so attached to the current fish now? ..is it time to prepare, have and use the Q-tank equipment?"
Once you decide to do it I completely agree that pip's example of a small bare-bottom as being the way to go. I've read a quite a number of Q-tank threads on the forum and to me, the best recommendations seem to be that you don't keep one running, you just have the tank, little heater and small filter equipment. Depending on your main tank situation, you either just keep an extra flter running on the main tank that you would move over (you would not move it back of course if the newbies turned out to be sick!) or, better yet, you just plan out a chunk of media that's going to come from your main filter and will be enough of a filter to run the Q-tank. My favorite of these are the people who say you can have a square chunk of sponge, say, in one of the trays of your cannister filter. You tank that out, jam a bubble tube through it and it becomes a simple sponge filter.
[If you've not seen one, they simply have an airtube (yes, you have to have an airpump) that fits to the "bubble tube" which is just a wider tube and the thin flex airtube runs down inside the bigger one, which has some side holes in it. When air bubbles up and out the bigger tube, it pulls water along with it and that sucks water through the little side holes. You jam the mature media sponge over top of the whole double tube with holes thing and then water has to be sucked through the sponge (the bio/mech filter!) in order to get through the little holes of the bubble tube. You could probably google a picture.] [Of course, likewise you could picture just using some ceramic media in a little bubble box internal filter type thing.]
Anyway, the important thing is that you plan it out thoroughly and have all the parts to put it together. Then you can just set it up quickly and only run it when you need to. Because its small, its also easy to water change it if the makeshift filter mini-cycles on you, no big deal.
~~waterdrop~~