Generally agree with the above, however they can and do survive more than happily in a smaller tank assuming they are fed appropriately- I have kept a single spotted mandarin in a 30 litre cube for a couple of years then upgraded to about 50 litres and then more recently into 100 litres, where my male lived happily in the 100 litres for over 3 years until last night when the 100 litre was shut down in preparation for our new tank. In fact I introduced the female just over a year ago so actually I had regular spawning mandarins in 100 litres....
They are however, fish that have very specific needs and they do need a lot of feeding. I managed in my small tanks more than anything because they were the the only fish. They have specific feeding requirements. In the UK most LFS brought specimens feed on frozen food, mainly Brine shrimp, whether this is the case in the USA or elsewhere, I cannot possibly comment upon. Foods have also got a lot better. In the 5 Mandarins I have had experience with (3x Spotted and 2x Green or Psychedelic as they are known in the UK) all have taken frozen food. All did/do take Lobster Eggs (bigger than oyster eggs and have a high protein count), Brine shrimp and Mysis shrimp. I quite often mash up a cube to make a more mandarin size mouthfuls but it is generally not needed. Our Green mandarins that we had in the old 4'x2'x2' tank would actively pick at whole prawns and would eat frozen scallop chucks as well- they certainly weren't the most picky eaters in that tank. Newly Hatched Baby Brine Shrimp is a good live food and stop gap however personally wouldn't rely upon it as a good long term protein source.
You can train mandarins to eat frozen aswell, often done with mixing live foods with frozen when feeding and slowly reducing the amount of live food until happily taking frozen. There is also the methods of using a feeding station such as the disk you can see in PaulB's video. Personally I have never used such a feeding station as have generally had a species only set up or trained to a pipette and target feeding (or both).
As for the masstick. There have been cases with Mandarins eating masstick, however it does seem a bit the exception to the rule. I do intend to try it with my mandarins in due course however I have not yet tried it. One method I have read is to mix in thawed and rinsed Mysis shrimp to the masstick and attach to a flat disc (such as a frag plug) and place on the sandbed. (I believe there is a you tube video out there somewhere).
They are certainly hard work but not impossible to keep even in small systems. It is however something that you will need to prepare for quite quickly, as previously stated they do need to feed constantly and will demolish copepods very quickly.
Good Luck.
