Dusty, you came here to look for help.
Every single person that heard you have a mandarin is telling you the exact same thing, "GET HIM OUT". Your tank is so unbelievably far from being appropriate for a mandarin it is ridiculous. Out of the hundreds of fish you could have selected, you decided to go with possibly the single worst fish (other than a Moorish Idol or Grouper

).
If you want help from the community here, you've got to show that you want to help yourself. Saying that your mandarin is eating and fat is NOT the right response. Even if your tank was 8 months mature, a mandarin wouldn't be appropriate for your tank. So having it in there after 3 weeks is certainly not helping your system stabilize, nor is it ethical to the poor fish. You should have zero fish in that tank.
Kathy gave you a fantastic list of things to do to help you get your system on the right track. You should follow every one of her recommendations. Then you should come back to Manhattan Reefs with the comment, 'I've done everything you said Kathy', 'What should I do next?'. We are honestly trying to help you. Whether you elect to recognize that or not is your own hangup.
Sometimes at this early stage of a system, it's less work, definitely less expensive, and much easier all around to take half a dozen steps backwards to right a horribly wrong beginning.
I hope you take the advice offered. It is quality advice.
Russ