As Alan says "ideally". However, Mandarins can surprise you by doing well in iffy situations. If it ate brine shrimp, which is basically like eating air
, it may take to Mysis, which is much, much better. If it was not skinny when you bought it, it may have enough reserves to last it until it takes to proper food. However, more live rock would improve your chances, .
but if you can take it back please do so