Um, the time to figure out the diet and the needs of the fish is prior to buying it. I understand that almost everyone in the hobby makes that impulse buy at least once, myself included, but when it happens, if you wish to keep the fish alive you need to do your research immediately and provide what it needs. A fish can decline in just a few days depending on when it last ate. Many wild caught mandarins haven't eaten since they were caught so they are already very hungry when you purchase them, this is why it is uber important to have a prepared tank ready for them.
Depending on whether it's wild caught or tank raised would determine how easily it would accept prepared foods as a guideline, although there are exceptions.
Mandarins feed on pods, it takes a large, established tank with a fuge, with alot of live rock in order to provide enough pods to support one. This takes advance planning.
"Taking It Back : NEVER!" is kind of a jerky way to look at it. It's not the fishes fault you didn't do research and now it is starving.
There is no "tomorrow", as thin as that fish is, something needs to be done now, if it's not already too late. From my experience, once a mandarin declines, they are very hard to rebound, and that is with ideal circumstances.
but fake rock :3 lul that mah comment NAO LOOK AT SOME PICTUIRE! lul
