Man, you have a habit of picking the worst animals for your tank!! :)
Boxfish are bad news in A) small tanks, and B) new tanks. Both of which is your situation.
I agree with Yote. You need to really read up on the animal BEFORE you buy it. Not after. And if there is a fish store you're going to that's telling you that you should buy these things, you really need to find another fish store. They are just telling you anything they need to to make a buck off of you. They are lying to you, and ripping you off.