My mandarin goby does fine and very healthy in my 20gal. with out a fuge and only about 20 lbs. of live rock. Also his tang should be good for a while until it reaches like 6 inches then it is time for a bigger tank like a 75gal. I also ahve my tang in my 20gal. but he is only about 1 1/2 in. but he will be moving to my 75 after it cycles.
10 gallon salt water 3 damsels
20 saltwater mini reek yellow tang green mandrin goby, clownfish, yellowtail damsel, coral banded shrimp, skunk cleaner shrimp, lots of corals and fiji rock
75 gallon cycling soon to be reef tank