I have a small 5-6 gallon tank with an aquaclear 20 hang on back power filter. It makes a decent -little- quarantine if you have rather small fish. Just requires some attention to water params. I use stress coat, stress zyme, and/or amquel+ to help keep ammonia under control. I suspect this would be fine for just a couple small clowns, a small wrasse and the like. If you expect to quarantine larger fish... tangs, etc. Then you need to upsize substantially.
Just a word of note, keep your QT tank items seperate from your display items. You don't want to accidentally introduce something from the QT.. and some of the nasty bugs you could be treating for could hitch a ride on an airline, pvc tubing, airstone... etc etc.