Think about it this way...
Take an average man on the street, say 6' tall, 200lbs. Have him sand in the middle of the floor, with his feet together. His 200lbs is spread out over a 1'x1' area (or perhaps 1.5'x1.5'). That's 200lbs per square foot.
Now compare that to your tank...
1300lbs seems like a very conservative estimate - I'd guess that the weight would acutally be less as water is probably the most dense thing in your tank. Anyways... take that 1300lbs and spread it over an area of 6'x2' (or whatever the tank dimensions are). 6'x2' gives you 12 square feet. 1300lbs over 12 square feet equals approx 108.3lbs per square foot. If you're floor an hold an average person, it should be able to hold the tank. If you are really worried about it, try to setup the tank so it runs perpendicular to the floor joists (across multiple joists), not parallel to them.