What did your LFS tell you about feeding it?
They require a massive amount of Copepods, a tank big enough to sustain such a population. Dragonets eat at least 16hrs a day. Continually hunting among the rock/substrate for these lil bugs.
They can be trained to eat certain types of prepared foods but this is very time consuming and not always successful. Still they would need Copepods to survive and thrive.
A slow moving fish it does not compete well for its food source. So other like fish are not recommended.