refugium is the way to go. i believe there are several companies which make HOB refugiums so that would be something to look into. mandarin gobies generally need a large amount of live rock in a well established tank to stay alive. most things i have seen on them reccomend at least 40 gallons simply because of their dietary requirements
on a side note, that 29 is way over stocked...the porcupine puffer is not compatable with the other members of the tank, sonn the small fish will become lunch. also, witha max size over a foot it will outgrow that small of a system