Not a good idea unless you have a way to cheaply feed it copepods. Or you could buy an ORA tank raised one for about $100 and hope it eats frozen mysis or brine shrimp. You need at least 75 pounds of live rock with a very healthy population of copepods and no other copepod predators in the tank to safely keep a mandarin. If you could keep copepod condos in a larger tank/refugium you could seed your small tank a few times a week for free. Otherwise you'll be going to the LFS and buying a small bottle of live copepods once a week for $20 a pop.