I have had long term success with my mandarins on frozen foods. Had my first for two years fat and happy until i got an angry trigger, he was returned eventually. Had one in my nano also fat and happy for almost two years, he then mysteriously died, i'm thinking my oversized anemone killed it, which sucks since i was moving it that day
I now have another in my nano and she has been doing very well, had her for about 4 months now. All three would eat mysis, brine, and bloodworms. My new one even samples flake foods sometimes. The nano does have amphipods in it as well which I'm sure helps keep them fat. My LFS
must have an awesome supplier since the mandarins he gets in always eat frozen foods.