What size is the qt tank? He may just not like it in there if it is small. How old is the rock in the display? If it is mature then put a few pieces in there for him to graze on. I kept my lion in QT for two months. I knew he was clean of parasites, but he was giving me a hard time eating. He was all of an inch, maybe an inch and a half. I had to 'play' with his food to get him to eat. Finaly I decided to add him to the display. He is a pig now. He always begs for food and swims in front of 'Sir' Quizzy to get the big piece of food. He never ate like that in the QT, and it was a 15L with excellent water flow and parameters. Certain fish just get tired of the QT and want a more natural environment. I would personally try him back in the DT. I would actualy catch the other YT and acclimate him into the QT for a week or two so that the larger one can regain his alpha status in the tank. That is what I would do. It is, of course, up to you though.