I do not recommend anyone not familiar with reef care keeping a Mandarin goby. They can be very picky eaters and are not aggressive enough to be kept in a community tank. You can get lucky and find one that will eat frozen food, but that is a trial or error situation which a responsible reef keeper would avoid anyway.
On the other hand I had a mandarin in my 29 gal reef for over a year, until i had to move. She was the only fish in the tank and the copepod population was in abundance. I never added any fish food to the tank since it is just a source of pollution in a balanced system. Without nutrient input, I didnt need a skimmer either and I still dont have one on the reef today, 3 yrs later.