remember that water weighs about 8 pounds/gallong US, plus the weight of glass, rock and stand, so your looking at about 500 pounds, at least. One major difference between a bed and a tank, is that a bed, I should think, weighs less, and the wieght is spread out more over the floor. A tank is pretty much concentrated on one area. Place the tank next to a load bearing wall and you'll almost certainly be fine. If you have any 500 pound friend, invite them to walk around the room and see if the floor sags or distorts. but 500 pounds isn't really that much in the construction world, so you'll probably be fine, but to be on the safe side, try to place near a load-bearing wall.
Hope this helps,
Brian Jagger