i am goign to have to disagree with both of the above statements...
very very few mandarins will accept frozen foods no matter how often they are introduced to it. very very few will ever eat it. if you dont believe me go to your LFS and see how skinny they all are. i have had one for about a ayear and constantly try. it isnt even interested in cyclopse eez (frozen copepods)
even swapping out live rock is not a good idea. it will take a while for your pod population to get back and you will quickly see how fast a mandarin will clean up a 15 gallon tank. trust me, ive seen it happen on a 29 when i first started.
and you can also order bottled pods, but that isnt realistic. once again, you will be spending 15 bucks at LEAST (probably more) ever week to feed this fish if you jsut keep buying bottled pods because of hos many the mandarin will eat. these things hunt for food all day long
just out of curiousity: how many pounds of lr do you have? and they all big pieces or small?