65 gal on 2nd floor question

[Aquattro]

Well, it isn't just one joist, as the plywood spreads over several and distributes the weight. If you can jump up and down on one spot and not go through the floor, it's fine. The weight/sq. ft. is not really all that much. If the place is really old and shaky, you might get some wobble in the whole display, but it won't go through the floor.