Very good looking fish. However, with the mandarins it is not the size that the fish grows to that is the problem, it is the pod population. It is unlikely, but hopefully it was actually eating the fish food you added to the tank. If it is not eating the food you add to the tank it will starve to death. I also tried to keep a mandarin once. I have a 75-gallon and at the time I had about 75lbs of live rock, this was about 6-months after the cycle. The mandarin quickly depleted my pod population and within a month and a half I could tell it was starving because its belly was sinking in. I returned it back to the LFS
in an effort to save its life. I hate to tell you but, I dont think its possible (unless it is eating fish food) for it to survive in a 10-gallon. Even if you stuffed it with 25lbs of live rock, you would still need a big refugium on there where the pods
could breed. I think the ethical thing would be to get the bigger tank, return it, or start to purchase live copepods in a bottle every couple weeks which cost about $20 per bottle.