I dont keep a Fahaka so I cant comment with any authority there. But the ones I do have (Hairy + Arrowhead) are very dependent on tank decor, not sure how a Fahaka would do bare tank. I know some puffers have a tendency to 'cruise the glass' if there is not enough decor to interact with.
I agree with an opinion towards animal comfort. Would this limit me to fish less than 14in??Your 18" width limits you a bit IMO. I would not get anything that wouldn't have space to go back and forth