Think about it this way...
If you took an average adult male who weights, what... 200lbs? Have him stand on your floor with his feet together. That's 200lbs spread out over 1 square foot of floor space. Most tanks weight a great deal more than 200lbs, but they also spread out that weight over a much greater area. As such, rarely do they have the same weight-per-square-foot ratio as a person standing with their feet together.
If your house is sound and in good structural condition, you should be fine. Be sure to run the tank across the floor joists so the tank is supported by as many joists as possible... but that's advice EVERYONE should follow, not just people with older homes.