Mandarin Dragonettes are very beautiful fish but they have a more specialized diet than a lot of other marine fish (this has led
to a high mortality rate in the pet trade for this fish). These guys usually only do well in really established reef tanks. Reason being is they will only eat live food such as amphipods, copepods, and other micro-organisms that will usually not exist, much less thrive in a fish only tank nor most recently established reef tanks (I'm sure there are exceptions here and there but not typically). That being said.....if you already have the Mandarin and don't have an established reef tank you could try live Brine Shrimp. If it doesn't take to that it would probably be best to try to take him back to the store you got him from. Otherwise he will gradually waste away.