if there is a load-bearing wall under the place you want to place your tank you'll be fine. if not, you can reinforce your floor. it's a complicated process if you have to do this (rip up floor, install new floor joists or add more, then replace subfloor, flooring and then set up your tank.
it's an even bigger pain if your s/o decides that your tank would look better someplace else to help your feng-shui... spelling error on the shway... damn japanese!!! yeah, so there are your options.