I just bought two mandarins a couple weeks ago. They had just arrived at the lfs, and looked like two females to me, but I purchased them anyway.
Upon getting them home, they were floating next to each other in separate bags, one flared its definately male fin at the other as they could see each other.
First couple of days they stayed at opposite ends of the tank, half-heartedly making attempts at frozen enriched brine that scooted past them. They never ate any. I noticed the movement attracted them and spent the next two hours adjusting flow so that any food flowing past the ends of my live rock (It is island style allowing swim room around all sides), gently and slowly flowed along the sand around the rock walls.
I then scurried to the pet store, and purchased frozen blood worms, and frozen mysis by Hikari. Pet store is exactly a minute and a half walk...which is a good thing as I had been sipping on some beer during the agonizing move of powerheads.
I tried the bloodworms first....and Eureeka!! I saw both mandarins eat at least 2 pcs each! Clumsy at first, they would move in on one...tilt heads, move closer...and if the food stayed in that spot long enough...success! At first when the food caught current and moved off, they didn't chase.
Now a couple weeks later, they have become quite adept at feeding, and will actually leave the corners, and successfully snatch food from the water column along with the other fish. It is quite a sight to see, as they seem like they are floating in mid-air, and then take chase to food going by.
They eat everything I throw in the tank, bloodworms being favorite, followed by mysis, and then the enriched brine. They even go for the flake at this point, but not as heartily.
Maybe I got lucky, but I don't see how I could have been so lucky with two at once. I do know that the current carrying the food past where they usually hunted was key. It looked like live food crawling across the sand. And about their small appetites, I don't see that...they eat and eat till food is gone, along with the other fish.
Will they still be alive in a couple months, or yr? I think so. I have my lfs on hand willing to take the female back if need be, as it seems my male likes to chase her around sometimes, or I will set up one of my 20 gals for just her until new tank arrives. They are currently in 46g bowfront until my new tank gets here. (If any of you have read any of my threads before I had a 150g on layaway) I cancelled it and ordered a 180g. Lol I would have gone bigger, but that's as heavy as I want to go.
Hope this helps anyone trying to get their mandarins to eat.