Mandarins are beautiful fish, but they are not as pretty as you start noticing that they are starving because they finished all of the bugs in the tank.
I have only seen two mandarins in good conditions, one in a 8 year old 200 gal tank and the other in a similar size tank and fed with live copepods frecuently.
I kept one for over a year, I was new in the hobby, when it started to get too thin, I set up a tank just for him, full of live rock, and fed adult brine shrimp daily, and even in this conditions, he never gained sufficient weight and died.
There is a a misconception that you can keep a mandarin and forget about it, they are not filter feeders, they hunt for food all day long. Same thing with seahorses, the first couple of months they do OK in our tanks, but soon they start to look malnourished.