Just as an initial aside - hope you're going with captive mandarins (I believe Biota is the main supplier on the market currently).
I got a pair about 6 weeks ago from Pacific East Aquaculture (I've been wanting a captive bred paid for years)... like other experiences you can find online, the fish are tiny and thin. In general, I QT all my fish for minimum 6 weeks utilizing different QT protocols (depending on the fish and vendor i.e captive only vs wild) but consistently with bare bones setup. For these guys, I setup a standard 20 gallon tank as a "grow out" home for them with live rock, sand bed, in-tank protein skimmer, and a selection of macro algae along with toadstool and gorgonian trimmings from the DT ... not a sterile QT with just PVC pipe.
Be prepared to keep them fed... I feed them a mix of frozen of baby cyclops (the cans from Algae Barn) along with frozen zoo plankton, baby brine shrimp, regular frozen cyclops, rotifers, etc (my DT has a lot of nps so all these frozen foods are always in my freezer). I feed the frozen food every evening, turning the power head off for 30 min to hour. I'm also adding copepods from Algae Barn every 3 - 4 weeks to maintain the pod population (regardless of how many pods I see on the glass).
Doing this, the fish have fattened up and look much better. Once they're bigger (maybe in 6 months, a year?) I'll move them to the DT... once there, I'll still add pods every 6 weeks or so (even with a 7 year old, heavily fed 125 gallon tank, I want to ensure there's always enough food for them). Currently, with the DT, I'm worried about the strong flow from an unprotected power head, the overflow they could easily fit through, and a marine betta (who, I've had since he was tiny and doubt would snack on them with how heavily I feed, is still a concern).
As long as your dedicated to making sure you provide enough food, I don't think the fish are difficult to keep... just make sure to go those extra steps in their husbandry and provide a good environment. Actively keep the pod population healthy and allow them plenty of time to eat when providing frozen food.
Of course, I've only had mine for 6 weeks so I hope my methods continue to be successful long term.
They really are lovely fish all around.
