Mandarins are extremely difficult to keep. And if you're feeding him brine shrimp, at least soak it in Selcon first. Brine shrimp alone is like you eating iceberg lettuce all the time--not a lot of nutritional value. A tank that small will not sustain him. Period. I've heard guys on certain other SW-related websites claiming you need an 8 FOOT tank with a 'fuge to have much luck. I'm sorry you were sold one, but the burden is on us as hobbyists to know in advance. So your options are either:
A) Take him back, and hopefully someone with a large enough mature tank will give him a home.
B)Set up a tank for him on your own and spend quite a lot of money not only on your set up but on buying him live copepods, amphipods, etc...(They arent cheap man. It adds up. Ask any seahorse enthusiast.)
C)Do nothing and watch your fish slowly starve to death.
By the way, from the look of his fin there, it's a male.
Good luck in any case.
--Ethan
