Myth #1: "According to the building code my house can only support a maximum total load of 40 psf anywhere on the floor."
No, the 40 psf is a theoretical uniform design live load over your entire floor. You might have a whole lot more than 40 psf directly under your aquarium, but that's okay because you didn't fill your entire room with aquariums either.
Myth #2: "So then, if I fill my entire room with aquariums that weigh more than 40 psf, my floor will collapse."
No it shouldn't. I said that the 40 psf was a MINIMUM design load and I also said that it is a SAFE load. That means that your floor could be (probably is) stronger than the 40 psf minimum in many places, and it also means that the full safety factor is still there to prevent a collapse.
Myth #3: "A structural engineer designed the floor structure in my home for a live load of 40 psf."
No, probably not. First of all, your floor was probably never actually custom designed. What builder would ever want to pay a structural engineer to design something this repetitive and simple. All an experienced contractor has to know is that 2 x 8's span ?? ft then he starts using 2 x 10's. See
http://www.mcvicker.com/resguide/page013b.htm (not my site!)
Myth #4: "A building inspector inspected my house or reviews calculations to make sure that homes can safely support a minimum design live load of 40 psf."
Maybe, but in many locations the building code is only concerned with public buildings. How strong you build your own private residence is of little concern to them. Of course, they still want their cash for the building permit.