I'll throw my situation into the conversation. I worry about this often.
I've looked into building codes and saw that most floors have a rating of 40ish pounds per square foot over the WHOLE area of the floor.... so if your space is 500sq ft in theory it should hold 20,000lbs of your stuff. But that still seems odd to me. There's usually a safety factor in those standards, let's say 2.5 or times so 100lbs or 120lbs per square foot. It's also got to be true that where there are tubs, fridges, water heaters then the floor is probably reinforced.
Now, my rimless 66 gallon is probably about 750lbs and the square footage of my tank stand is 5.43 (it's in cm, 100cm long by 50cm wide) which means approximately 138lbs per square foot of tank. Although I am over the psf, I know my second story apartment floor is concrete, and is probably reinforced. The stand has a solid piece of plywood bottom and it's in a corner with an outfacing wall.
I still get worried, but it's been there going on two years. Does all my rambling check out? Do the building codes tell us anything? since that is what I am basing my math from..