I've been lurking around RC for a long time and finally decided to make an account, so this is my first post (sorry if I end up breaking some weird forum etiquettes or something)
I've had my mandarin for 4(or 5, lost count :rollface

years on frozen only(there aren't that many pods in the tank), and he is easily one of the fattest little buggers I've ever seen.
Disclaimer: Now I am NOT recommending everyone that reads this to go buy a mandy and keep it in a nano. This was just my experience, and I have heard very few cases of this being successful. I don't know if my success is just a lucky outlier, but for those with mandarins feeding heartily on frozen, I believe a smaller tank with slow or small tank mates is best to ensure that the fish gets a sufficient amount of food.
Here is how I did it:
Selecting:
When selecting at the store, you have to pick a mandarin that has NO SIGN, whatsoever, of skinniness. It also has to be interested in whatever small particles pass by it in the tank i.e, if something small passes by it, the mandarin actively chases it.
My training experience:
For training, I took a 10 gallon tank, hooked up a simple tetra hang-on-back filter, and made sure there was sufficient flow as well as a few dead spots in the tank. It already had an actively eating cardinal or chromis(can't remember lol), which may have helped the mandarin figure out what food was. I bought hikari baby brine nauplii(frozen), and tossed in a cube every morning and evening, the assumption being that nauplii look almost the same as pods. I made sure the mandarin was pecking at it and actively eating it. I then added a few frozen spirulina brine shrimp to the feeding every day. It eventually took interest and occasionally pecked at the shrimp. After about 3 days, it was heartily taking frozen and I stopped feeding nauplii. I tried to also get it onto mysis, but it hated mysis with a passion, and I was never really successful.
The tank:
I actually left the mandarin in the 10 gal for a year. Then upgraded to a 20 long for another year. Now it is currently in a 36 bow front, but soon will likely be going into a 15 gal. The important part of the tank is to make sure there are dead zones in regards to flow in easily accessed areas in the tank. This will give you a place to put food and give the mandarin time to eat.
Tankmates and Feeding:
currently, the mandarin lives with two garden eels and a yellow clown goby. Essentially, the tank mates must not move a lot and be able to steal food from the mandarin, or they must not eat a lot during feedings. For feeding, I squirt a generous amount of spiraling brine into one of the dead zones and just let it sit. The mandarin does not eat all of it at once, in fact, sometimes it takes half an hour or more to eat his fill. The thing is, I don't remove the extra food. I understand that it can foul the water a bit, but that is easily remedied with either water changes or a good skimmer/filter. I also have one nassarius snail (just one small one, big ones or multiple ones can sometimes beat the mandarin to the food) to take care of any extras.
other notes:
I have tried and tried again and again to get the mandarin to eat frozen mysids. He only began somewhat taking them this year. for the last 4 years, he had been living off generous amounts of spiraling brine shrimp. He absolutely refused other foods. I chose the spiraling brine over regular because at least I knew he was getting some extra nutrition, in contrast to the relatively nutrition empty regular brine. The main thing is to make sure he has enough in the tank, and that you don't manually remove extra food.
Anyway, I know its long, but this was my experience.