I have had 55 gallon tanks warp the floor of 'normal' houses I have lived in. I would go perpendiculiar to, not paralel to any joists, so that the weight gets spread out some.
Do you have a bathtub up there anywhere? Think about that for a minute - those things hold about 50 gallons of water in a 'normal' one, and you never hear of somone's bathtub crashing to the floor.
I got hell from my wife when I was talking about putting a new 200 gallon tank in our house. As soon as I reminded her that the garden bath tub had to be about that size and has never crashed into the basement, she loosened up to the possibility some. I don't have the tank yet, but at least I don't have to listen to that crap anymore.
Heck, you can always go to the Wallyworld and buy a half dozen 50 pound weights for a weightlifting bench. lay them on the floor where that tank will go...... If the floor holds, set it up!
Your dad might not like that plan, however, so you are on your own.