You could run all of the fish in QT together just using hyposalinity. However, as Mebbid said, unless the mandarin is eating frozen or pellets, he will likely starve in QT for lack of pods
to hunt and eat. Unfirtunately, there's no real good solution for that particular fish.
If you have no inverts, you could do hyposalinity in the DT for 6 weeks and kill the parasite that way as well. You'll still be in the same boat with the mandarin, however.
EDIT: Nevermind the hypo in the DT. i just looked back at the photo and saw the mushrooms.