My suggestion is to return the fish to the store. Blue tangs grow to 12" and require a 125 gallon aquarium. They should always be housed in a QT tank for a month before being introduced to the main tank. That actually should be followed for all fish, but the Blue and Powder Blue tangs are called "Ich magnets".
The tang is a herbivore. It needs nori (seaweed) as a diet staple. It also likes to hide in LR
, so you would need to at least triple the LR
you have in the tank.