Tank weight
As has already been stated, the tank must be supported all the way around the edges. It must also be level. If the floor is unlevel, such as a hump or dip in the flooring, it will cause the stand to flex and stress the tank. Also, the top of the stand must be level...or true might be a better word. If it is off just a little, you can put a piece of styrofoam sheet under the tank ( a thin piece...maybe 1/4 to 1/2" thick) and this will take up for the imperfection and distribute the weight evenly. If the floor is unlevel, shim the bottom of the stand so that it sits level and doesn't rock. This will probably not cause the tank to crack, but increases the chances of a leaky seam especially with the weight of a reef tank.
Logan J