You really have to have a well established, larger tank with LR and even a refuge attached. Trying to "get" the food sources you would need to maintain a goby is not realistic.
Mandarin's are awesome fish but it comes a time when you must just understand that you need to suffice with another fish... first of all it is a must to have a fuge if you want a manderin secondly I def. suggest having experience with other fish first... believe me they are very picky eaters... the problem with these beautiful fish are they starve to death most of the time if you do not have enough for it to eat... they are right out pigs and will eat your supply before you know it... I wanted one for the longest time but it is not possible with a 30g.. sorry