I agree with Brandon wholeheartedly on this one. I understand they are a business and need to sell things, but selling something like this to someone and not advising them about their diet or care is not right in my book.
But the majority of the blame should go to the buyer. This is why impulse buys are a big no-no. If you want to buy something and don't know anything about it, then you should go home and research it first and find out if you have the proper environment and equipment to house the new addition and have him thrive and be happy. Then make your decision as to whether to buy it or not.
I'm not chastising you or anything, but this is a perfect example of what not to do. Knowledge is the key to a successful reef tank. Ignorance, impatience and impulse are the enemies.
No one should be buying anything and then posting on here asking how to take care of them. That's not responsible reefkeeping.
Please, in the future, be more mindful of what you are buying and know if your tank is capable of supporting it or not.
IMO, your best bet is to return the fish ASAP and tell them you don't have a suitable environment to support the mandarin.
Hopefully, this will help you better understand the needs of the livestock you buy and will enable you to provide the proper care for each of them, so they thrive and live a long and healthy life in your tank.
Good Luck.