Mandarins require live food, copepods to be specific, which grow mainly on your LR, but also in the sand... Areas of lush algae are wonderful places for them to hunt. A copepod population sufficient to sustain a Mandarin takes about 8 months to become established on about 100lbs of LR...
I've had my Mandarin for about 8 months and he eats constantly, infact he's huntig a pod as I type this... The leading cause of death among Mandarins is starvation, so please, if your tank isn't at LEAST that old (or if the LR comes from a non-established source like mail-order), don't even consider a Mandarin yet, cuz it will die... Some people claim to have one that will eat prepared foods, I wish I were that fortunate... I've never seen one that would, and I doubt you will either..