Mandarins (they are dragonets, not gobies) eat almost exclusively live microcrustaceans (pods). They generally need about 75 lbs of old, seasoned live rock to thrive. They are also not only peaceful fish but exude a toxic slime. I consider them one of the few "most pessimum" fish to put into a mantis tank.
A six-line wrasse is a vastly better choice for this sort of thing. Mandarins don't eat worms!
Dan
