[pogodzib]

If you look at the weight of a tank and compare it to how many people equal the same weight over a certain area or to how much a house weights per lineal foot of bearing wall, you will see that a tank doesn't weigh that much, BUT, since it stays in one spot, it can be considered a "dead" load, whereas will people, it is considered a "live" load since the load moves over relatively small time periods. Dead loads are more critical when it comes if your floor will end up deflecting over time. IMO, it is always better to be safe than sorry and you should verify that your house can handle the weight.