They will commonly do this but you really should provide it also with the algae sheets or herbivore preparations. I would say, and its not offense meant, you have been exceedingly lucky so far. Best of luck with the copperband which is notoriously difficult to get to feed and live. Mandarins, also, tend not to eat anything prepared or frozen and typically will starve without lots of LR or a refugium system. To add on to the brine thing, if you feed live brine you can even get some liquid foods (such as selcon or similar) and put it in with the brine...let it sit some time for the brine to "eat" and then the mandarin will get some extra food. The tang, IMO, looks quite good (as do the other fish). With the addition of some more live rock, and maybe a lighting upgrade (which is typical), you may be ready to start adding some corals!
What are your nitrate levels like?