Hi Rene and welcome!!! Good to see you over here!
If you're referring to Synchiropus stellatus , (Starry Dragonet), it is not an actual Goby. They're a different species, and commonly called Scooter Blenny, though they aren't a true Blenny either. Dragonet is a family, all it's own, with some very specialized food needs. They eat micro fauna in the tank, which is animal life, as apposed to micro flora, which is plant life. So, you're particular Starry Dragonet will eat pods. Copepods, Amphipods, small mysis shrimp and other tiny Zooplankton in the tank.
They need a very mature tank with LOTS of live rock. They usually won't eat foods that we offer them, and instead need to hunt and peck at the live rock, eating what grows naturally, in our tanks. Because of this, I wouldn't recommend one in any tank less than a 46 or 55. You need this amount of live rock, in order for their natural food to reproduce, in a high enough quantity for them to be sustained.
It's also important to NOT have any other fish that will compete for the same food source. Since Dragonets are SLOW and deliberate eaters, other fish will out compete with them, for the same food source. Common fish that will create problems are, a lot of wrasses, hog fish, and other small carnivorous fish.
Scooter Blennies have the same needs as Mandarin Gobies (also not a true Goby) aka Mandarin Dragonet. You'll find a lot more information on Mandarin Dragonets, online, than you will Scooter Blenny. Since the care and needs are almost identical, if you find information on Mandarins, it'll carry over to the Scooter.
PLEASE, ignore all the sites that tell you this fish will do fine in a 20 gallon aquarium. They wanna sell you a fish.
Here's some great information.
http://www.wetwebmedia.com/mandarins.htm
http://hubpages.com/hub/Mandarin-Fish