Stage
Building a "stage" or using plywood under the tank isn't going to do you any good (IMHO) unless you make it much larger than the tank itself.
Basically, unless it is a old house/apartment their is plywood (3/4" typical) underlayment already, it is going to distribute the load over the joists, of course the more joists the better.
I'd cruise down to the basement and see what size joists they used there, more than likely they used the same size throughout, 1x6 (move out now), 1x8 (marginal at best), 1x10 (nicer), 1x12 (stick that puppy in .. two even). 16" on center is the standard. Are they using plywood underlament? (good) or lumber, e.g. 1x4 or ? (OLD building .. not so good).
Placing the tank over a load bearing beam would be the best solution. Next best would be up against a load bearing wall. And obviously, positioning the tank so that it was over as many joists as possible.
And you really can't equate waterbeds with tanks, the distribution of weight is over a much larger area than a tank.
Finally, I forget what the load factor for floors is .. but if you call the building dept they should be able to tell you what code is or was at the time the building was constructed (assuming it was after codes where created and enforced).
Just my humble take on the matter.
Regards
Noel