Sep is correct in her assessment here. We cannot provide the nutritional requirments to keep these critters long term.
From U of M Museum of Zoology, aminal diversity site
Lifespan/Longevity
In the wild mandarin fish are expected to live between 10 to 15 years (Sale, 2002). In captivity, however the lifespan is greatly reduced due to dietary requirements. On average, mandarin fish live between 2 to 4 years in captivity (Delbeek, 1989). (Delbeek, 1989; Sale, 2002)
And from wetwebmedia
"It’s not unheard of that a dragonet will accept (with training) frozen/defrosted foods in place of live, but it is rare that non-live foods sustain them. In the wild their food choices are principally small crustaceans and worms. You can culture these "incidentally" in a large main/display system with lots of substrate and/or live rock, but adding a live fishless refugium will go an immense distance in assuring your mandarin/s receive sufficient live food. These fishes cannot live on dried-prepared or frozen/defrosted, or chopped meaty foods."
From any reading I have done 2-4 years is unheard of in systems that;
Do not have a fishless remote refugium to cultivate live foods.
Have any other predatory fish competing for the same food stuffs as the dragonet.
So you can see these creatures are doomed, as probably most creatures are, in our captive systems, but their life spans are greatly reduced where others at least have a fighting chance.