I am a newcomer in the saltwater scene(had my tank for eight months) and I have read alot about the Mandarin fish. Ninety-nine percent of all types of different sources have said the same thing: need a well established pod population, need at least 75lbs of live rock to thrive, and will most likely not accept prepared/frozen food.
HOWEVER, two months ago I spotted a starving Mandarin(sunken belly) at a fish store and even though I knew it is not wise to purchase the Mandarin beause of the above mentioned reasons I did buy the Mandarin(figured I couldn't do any worse than the LPS). Note: the fish store had the Mandarin in the tank with maybe 10lbs of live rock. I did alot more research and found that in a few cases the Mandarin fish may take Mysis shrimp, brine shrimp(not a good idea), or blood worms. So, I tried the Mysis and at first the Mandarin would only eat a tiny bit, but now will eat as much as I will put in the tank and is starting to look alot better. KEEP IN MIND that this is only one experience and I still am not sure how long my Mandaring will last
Also, another fish store I went to said that the green spotted Mandarin is easiest to keep alive and that is the kind of Mandarin I have