Okay here is what I know is 100% true about Mandarins and isn't opion. They need a minimum amount of 75 lbs. of live and a good sized sand bed (like 4", maybe 3"). Although you could always have a refugium also which would sustain what they need or buy pods online but that is expensive. As a lot of people know on the forums all to well is that Mandarins need lots of pods for their food source. They can and might eat brine but that isn't good enough to have them live of to their average life span. Their mouths and eyes are designed for eating pods. My tank is 40g with 75 lbs. of live rock and 90 lbs. of live sand. With that set up I wouldn't even buy one. But I have a refugium that has an extra 16 lbs. of live sand and 16 lbs. of live rock. But even with all that the 91 lbs. of live rock, 106 lbs. of live sand, and the refugium I wouldn't get a Mandarin until my tank was set up for a year atleast. They are very awesome little guys and that fish is the reason I started this hobby because I saw a picture of one in the 5th grade and I fell in love. I love them but not to death. I will have one someday but paitence is key with all saltwater fish but is extra special with this fish. To ignore its speacial needs to me is disgusting but I am not pointing any fingers at anyone here. So in short if your tank has at least 75 lbs. of live rock and a good sized sand bed and your tank has been up for a year go for it. So good luck and I would take that Mandarin back to it doesn't die/suffer.
:happyfish