Easier math. Multiply the gallons by 9lbs. Water weighs 8.64lbs per gallon. I'd even suggest using 10 as a multiplier as that ought to cover the stand as well. USMC is correct. Place the tank near a wall, especially a corner. More then likely you'll end up placing the tank perpendicular to the cross beams under the floor. You should even be alright running parallel. My home is near 100 years old, built out of 2x4 running 20 feet between tiny tree stump piers. I have a 75g running with the joists and a 125g across the joists. You can just as easily brace the floor with the addition of some 2x4 or 4x4 studs wedged under where the tank will be.