When I set up a QT I use some water from the tank the fish will be going into, and steal some filter media from that tank too. My QT is 25 litres and I use about 8 or 9 litres of old tank water, the rest new.
I don't medicate unless there is something that needs medicating.
You'll find varying lengths of time on-line for keeping fish in the QT. The most common thing for new fish to come down with is whitespot. Two weeks is enough for this to show. But for things like intestinal worms, the fish need to be quarantined for a couple of months - discus owners quarantine that long for this reason.
