No problem. It can be frustrating. Do not always believe the fish store. Sometimes they know what they are talking about and sometimes they don't. Same with the forum, except we don't get paid, so all we want to do is share information.
Box fish are a advance aquarist fish that can kill other fish as it dies. I made the same mistake. He was so cute in the store, but I didn't have a clue. Nano's are very delicate. You need to be very very conservative as to what you try to keep in it. Especially at first. Get the box fish back to the LFS
. Get some gobies, or blennies, maybe a shrimp or two.