I've made this point before, but I can't believe that any store would sell a stand for a large (55g+) tank that has only 4 or 6 legs that come in contact with the floor. A typical 90g tank weighs about half a ton, and if there are six "feet" on the stand, that means 160+ pounds per very small point. A normal human weighs at least that much divided by two feet which are each several times the surface area of one of those "feet". And while my house is only 65 years old, I (and the surface of my tanks) feel vibrations from a person walking across the room.
Do yourself, and your joists, a favor and build your own stand with as much floor contact as possible. Perpendicular to your floor joists where possible. As long as you use a level, and a whole lot of screws, you can't possibly make a less sturdy stand than you'd buy at an average LFS unless you were drunk, blind, and trying to destroy as much as possible.