twoods is absolutely correct..Mandarins are pretty tricky to care for. I received mine only because he was doomed in a LFS that had no clue whatsoever on how to care for him. I have been reading as much as possible on them and as stated above, copepods are the primary source for food. If you have a nice amount of live rock/live sand, you should be able to look at it during the night when your lights have been off for a couple hours and see them running around your rock/sand.
You can also buy culture kits for copepods from online stores such as
http://www.inlandaquatics.com/
But the best way to do it is to produce your own. I am NO expert on Mandarins but from what I have read, that is the best way to care for them. Also make sure you don't have other aggressive pod eating fish, Mandarins do not fare well against competing fish and may starve. I did read that if your Mandarin does take to brine, it can also mean that he is close to starvation and is just putting something in his stomach. You can also go to a LFS that sells live rock and ask them for some "rubble". That is the residue that falls from live rock and it is usually infested with copepods, drop some of that in there and hopefully there is enough to not only feed your Mandarin, but maybe seed your tank enough to maintain a supply of food for him down the road.