The mandarin wont chase down food in your tank. You might have luck if you could drop food right on top of the fish. The frozen food needs to move across the surface of the rock in order to stimulate the feeding response. I have had luck getting a mandarin in a fairly small tank to eat frozen blood worms, brine shrimp and an occasional tiny mysis shrimp. They are very near-sited and wont see anything that isnt right in front of their face.