not to beat a dead horse, but there are too many factors to cinsider in this. Size and spacing of floor joists, bearing points below, location of tank compared to bearing points, and on and on. (by the way, I design houses so that's where this comes from). Things like age of building and location of building would be big factors, because that would tell you more about the codes enforced during construction.
As a side note, if you live in a newer building (10ish yrs) I'd say you are fairly safe. Most buildings that age could hold a waterbed safely.
Please check with landlord. Just cause floor can hold weight doesn't mean he will let you, and I had a few landlords say after a certain size tank I had to have renters insurance.