Most people for a QT tank will have very little in it. No sand or substrate at all. No rock either, instead cut up large diameter PVC
into short pieces the fish can hide in. Most people will use a small HOB
filter or a sponge filter. If you have a sponge in any of your filters now i would put that in your QT tank in an area of good flow since this will provide some bacteria to try and keep it from going through a cycle.
You should be doing water changes as need (which may end up being pretty often). Keep a very close eye on your water chemistry as it will change much faster in the smaller water volume. Check frequently for ammonia to make sure it doesn't start building up.
If you get a sponge filter for your QT after you are done treating the fish you should clean and let the sponge dry out (or just get a new one sinc ethey are pretty cheap) and put it back in your sump. This allows it to build up a colony of bacteria so the next time you set up your QT you know it won't go through a cycle.