Its impossible to say.
Just for info, im an architectural technician, so i work in construction design.
I did calculations a while ago, and a floor designed and constructed to minimum standards in both the UK and USA will support about a 75g aquarium. In practice nobody designs floors to the minimum, there will always be some overdesign of at least 50%, more likely 100%. In addition a floor will support more load if the load is close to the perimeter than in the centre, and will support more load if its perpendicular to the span of joists than parallel, because it will span across more joists. So on the face of it, your 40 to 50 gallons is fine. And many people keep aquariums upwards of 200g on upper floors.
But, you havent specified if the home is built to modern standards. You havent mentioned if the build quality is good. Ive lost count of the number of times ive seen too large holes cut through joists to route services or not constructed to specification to save money. Thats a reason why we overdesign because we don't always have control over build quality and need to allow for the construction team to cut corners.
So. You are almost certainly going to be fine, but you may not be if the floor is substandard for whatever reason. I cant tell you how risk averse to be. But would you be concerned about the floor if you had 4 or 5 friends over and they all stood next to each other? Maybe they enjoy a game of Twister? Its no more loading than having a bath installed.
