Sorry to hear about your black boxfish,
My Dog Faced Puffer is very mellow and no one pays him any attention really. He has puffed up twice that I have seen, when he does he is very intimidating to everyone in the tank. BTW it is not good for them to inflate, they can sometimes have difficulty deflating. The two times mine inflated I could see no reason for it. Once was after lights went out I heard him splashing around at the surface. Got a flashlight to see what the problem was, no idea so I just did what I do every day and pet him through the glass. He responds well to this... You got a good deal on yours, mine was like $40. When "Chompers" gets stressed like during water changes, he turns dark and lays at the bottom of the tank until I am done. Not sure this really answers your question but its info for the future.. Long live your puffer..
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110 Gallon FOWLR, 25 Gallon sump/refugium.