I would always suggest running at least a Tigger Pod culture in a 5-gallon bucket or bussing tray. There is simply no better food to get finicky eaters to fatten up with. They are easily cultured in old tank water and if it isn't too cold or hot where you live you can culture them outside.
Next in line would be a brine shrimp hatchery. That is actually the easiest live food to offer, but unfortunately not the most nutritious.
Finally, you should try to raise some brine shrimp to adults. Live adult brine shrimp are the best way to train difficult fish to recognize them as food and then also accept frozen brine shrimp.
Using those feeds I managed to rescue a bullseye mandarin male who was literally just skin and bones and ultimately got him to stuff himself so much with frozen brine shrimp that he looks like a gravid female.
BTW, with mandarins diseases are usually my last worry - their skin is too toxic for the usual suspects (Ich, Velvet, and "flukes"), and what they can't poison they just slime off. Picking one that is looking healthy and who is searching for pods and then getting them to eat frozen is usually all that is needed with them.