Mandarins rarely will eat processed food (frozen/flake). they prefer their natural food...live copepods. these pods live and multiply readily in a healthy established (usually a minimum of 6 months) reef tank. Most people find it easiest to grow these 'pods' in a sump/refugium where there are lots of macroalgae (usually chaeto).
If you read the description of these fish on most internet sites it will indicate that they will slowly starve if they do not have these pods.