Did research on this as I am building a 240g, so here it goes...
A normal residential house floor is rated to ~40-50 lbs/square foot. This number is across the entire floor and not necessarily in one or two spots in the floor. The individual boards spread the weight evenly out through the room and to the walls and then down to the structure of the house. This is how a 200-300lb person can stand in a square foot area and jump with no worry of falling through. But it gets complicated when you take a large tank into effect as 1g of water = 8.9lbs. so 300g = 2670lbs, plus weight of substrate 4'x4'x3" depth = 375lbs of sand, include tank weight 300g plywood with 1 window ~200-300lbs with stand, sump tank? say 75g = 667lbs plus tank ~100lbs (glass). We'll stop there for simplicity but that all works out to ~4112lbs on an area around 5'x5'? = 165lbs / square foot with an area of 25ft.
Common misconception is that if you exceed the floors structural capacity you will fall through like a fat man in a bathtub, but in reality it will damage floors and structural walls etc etc. It may never fall through but it will cause damage without proper bracing.
A great way to think about it is if you pick up a 150lb item, this exceeds your normal lifting capacity but you are able to hold it up, now it's not whether you can hold it up for a few seconds but rather how long until your arms give out and you let it go. Think of this in the same way as the floor boards, yes it can hold that 4 ton tank, but for how long?
My recommendation is unless you know a civil engineer, put it in the basement. (~1000-3000lb/ ft^2 capacity on a basement floor)