Its better to have a quarantine tank(QT) for new fishes. Cos you will never know wat dieases or parasites the new fishes are carrying. Observation can be made in the QT.
Usually the quarantine period is abt 2 to 3 weeks the min. Given the time, any illness or parasities would have shown itsels already and proper medication can be used. eg copper which can 't be used in a reef tank. Usually all the medication which are not reef safe are the most effective.
Getting the fish to feed in the QT is easier as there is not competition.
the set up for the QT should be simple. all you need is a pump\filter and a few PVC for the new to hide when they are stress. No need to put in sand rock etc cos the tank is only temperory