No not necessarily , in the end it's up to you. A lot of people ask for what fish for this size tank or that size tank. I feel it's up to me as a fellow fish keeper to give my best opinion as to what the long term care for an animal might be.I take it a fahaka puffer just isn't going to be ok in a 125? So he is out all together?
The width of the 125 and the possible full grown size of the animal is to closely matched to say.
Some will say that you can keep fish in smaller than recommended aquariums as long as you do the corresponding maintenance.
Seriously Fish says min tank size for a fahaka is 48x24x24 which is 120 gallons
Its the tank dimensions that are critical in this situation.