Using reason like a tank puts less lbs/sqft than a person or other house hold items isn't good. By the same reasoning you could say a floor can easily support 200lb/sqft since my buddy is 200lbs and can stand on a single tile meaning you could install a swimming pool without problems. However it obviously doesn't work that way and residential buildings are typically rated at a minimum of 40lb/sqft.
Without knowing the exact details of the floor support you can calculate how much weight you can put in a room using the standard residential minimum of 40 lbs/sqft.
For example if your room is 20'x10' that's 200 sqft meaning that room has to be able to support 8000 lbs total. Next apply some reason as the tank won't be distributed over the entire floor. Say for example the tank runs perpendicular to the floor joists and you manage to get the tank to sit on 6 of the 10 joists in the room, therefore using about 60% of the support. So you don't want to exceed 60% of 8000lbs or 4800lbs. That of course is max so you'll want to subtract for other furniture and live weight such as yourself and other people.
If you do know details you can play around with this