In our recent condo, I had a 60 cube which was fine in it's initial spot in the kitchen.
I had to move it to the third floor, right near a staircase, to redo the floors downstairs.
Anyway, the floor started to sag. What I noticed was a split in the drywall seam between a half wall that separated the staircase from the room, and the nearby corner. I left it like this anyway for a month and simply repaired the seam and moved the tank ASAP. In the end, no apparent damage done, but, over the long haul would have been a bad spot for the tank.
I also had a 135 gallon tank which was moved from rental property to rental property with no regard to any floor or potential damage (I was young and stupid), and nothing bad ever happened.... but... if someone walked too fast near it, you could often see it rock a little bit!
My current 300 is under construction and I completely rebuilt the joists under it to be sure it wound sag, rock, or have any other wt. related problems.
I guess the bottom line is that without knowing the joist structure under the tank, it is probably a crap shoot.
Ryan.