Craig,
You hit the nail on the head when you say that without some understanding of the structure and it's dead load capacity, you won't know. Having said that, I've seen alot of tanks this size in NYC apartments...especially in older buildings that may have been used as heavy commercial spaces years ago, they have a higher dead load capacity.
Speak with the super, but don't get into too much detail, most places will give you a hard time with a tank that size.