weight limit on 2nd story floor
Your immediate area is going to have a load of a little more than twice that. Considering safety factors built in and the fact that the whole room will share the load, not being as highly loaded, youy should not have any trouble. If you are aware of which direction the floor joiusts run, it would be best to spread the weight over as many as possible and to place that load near the ends of the joists, rather than in the middle of the room.
The designed load for a bedroom or attic only has to be at 30#/sq. ft. instead of the 40# Piffin mentioned (which is the case for the other rooms). This does not necessarily mean yours are designed that way, but without knowing the span and size of the joists there is no way to tell. Keep the aquarium against a bearing wall and perpendicular to the joists and you should have no problems. (I have a 135 gallon aquarium in my house.)