A 20 gallon QT is fine for this fish. A couple potential problems; it's a small specimen and sometimes at this size they don't acclimate well. Offer it some good meaty foods. Mysis shrimp, finely chopped mussell and clam, Marine cuisine, Emerald Entree', sponge based foods. Agree with the nori. If there's no live rock in the tank and the tank is not being medicated or in hyposalinity, place a couple pieces of rock for grazing/added shelter. A healthy fish should be curious about different foods and should eventually eat just about anything.