serious things you need to think about....
location of load bearing walls.
direction floor joists are running.
foot print of aquarium.
keeping these things in mind, you shouldnt have a problem with structural issues unless your hous was built prior to 1970 or so. The psi isnt a big issue as long as your tank isnt placed directly in the center of the room below it. If by chance there is a wall directly under the tank, then you will have no issues period. you will even have a great spot to run plumbing to the basement for a sump if you so desire.
one thing that stops most people from putting an aquarium on the second floor is the fact that dragging all that water up stairs to do water changes over a period of time will make you reconsider the second floor. If its a 10g aquarium, then you wont even think about it. but when you get to bigger tanks and bigger water changes.... that 50 gallons of water in 10 jugs of 5g each will equal about 5 trips up and down stairs just to lug the water up. Also think about the problem that if you plumbing breaks and pumps water all over the floor, you are not only damaging the floor, but the ceiling of the room below, its floor, and whatever else it may come in contact with. And let me tell you...... replacing drywall on a ceiling really sucks.