First I don't have a Mandarin yet but once my tank gets going I will have a pair at least.
I have seen people swear that you can't QT a Mandarin but I have seen some that say, "the QT is the best place to train them to eat prepared foods." Basically you need to make your tank pristine and not treat for anything, you are just watching to make sure that the fish is healthy. It is suggested that you have live pods
that you can add to the tank (live brine shrimp is one that you can hatch yourself) If your fish is healthy but you can't get it to eat anything after a week then you should put it into your main tank but if things are going well you might be able to train it to eat frozen or dried foods.