Yeah, there have been quite a few posts on here about failed attempts with mandarins, usually around the several month mark, where they've starved to death. While they can often survive on frozen food, it's not easy getting one to eat it. I have a 180g tank and my mandarin only eats pods, all day, every day. He just cruises along hunting for pods all the time. If this same mandarin has ended up in someone else's nano tank, it surely would have died of starvation by now. So if you've managed to get yours to eat prepared foods and it's survived for quite some time on that, congrats!
At any rate, the tank looks good, I love purple firefish, and hope you can make out with your mandarin. Welcome to the boards, this place has a wealth of information, but there are some people who will flame for time to time.