I'd suggest building some sort of secondary containment, but it would get pretty big/difficult for a tank that size. Easy way is to take a 4' x 8' sheet of plywood, put 2" x 4"s on edge along the perimeter, and seal it with epoxy or put in a vinyl shower pan liner or something. But that only holds ~60ish gallons, and even using 2"x6" will only get you up to about ~100 gallons.
Anyways, even if the floor is okay, if you get a leak, that water is probably going to get in the walls, and potentially cause damage.
Also, does your lease specify anything about waterbeds? (whether they are specifically permitted, not allowable, allowable with permission, etc.)