I have a bunch of architect friends and they always tell me to just do the math. Seawater weighs about 8 1/2 pounds per gallon (or so I've been told), so figure for a 90 you've got around 700 pounds. Add in the weight of the tank, substrate, stand, filtration, etc. etc. and you're probably around 900 pounds or so. The floor joists are going to be running in a certain direction, so if you place the tank perpendicular to the joists which should be about every 16 inches (if I remember my code correctly), you're not putting much more weight on a section of your floor than one or two people, and you don't get nervous about standing in one spot on your floor for too long, do you? Just make sure you place the tank perpendicular to the joists, and you'll spread the weight out evenly. Even better, place it near a load bearing wall and you'll be fine. If you're still nervous, call the architect or go to your city hall and ask the building inspector how much weight your building can handle based on the plans filed with them.