Agree with the above advice from Josh and Europhyllia. Mandarin's are known to be harder to keep, especially wild caught ones. They don't do well in smaller tanks unless they are already eating brine. In your smaller tank, and even in the store you bought him at, he may not have been able to find food. Your 75 should be big enough to support mandarins once it has been established for a while, 6-12 months in a rough estimate.