That is a really nice tank.
120 US gallons.....drool! I have a 48"x12"x24", 60 gallons. I wish I would have held out for a tank like yours. The extra 12" of width really cranks up the gallons. Please keep us updated as you progress.
Edit... I read about concerns with the weight. Just with the
water alone, you will have approximately 1000 lbs. The area where you have the tank, are those 2 outside walls of the house? If so, they both will be load bearing, meaning the floor joists should be adequate to hold that weight. I would only be concerned if your total weight is closer to 1600 lbs. If those walls are both not outside walls, and the one directly behind the tank is an interior wall (non load bearing). You might have some concern. Especially if you are on the second floor. You may want a structural engineer to come an look at your flooring frame work. If you do not have any engineering friends, you could always look in the telephone book. What has me concerned is you said there is some creeking of the floor boards. Better be safe than sorry. I really don't think it's cause for concern IF those two walls are outside load bearing walls.