A properly constructed stand will support many times the weight of a tank full of water. Typical construction grade lumber has tremendous compressive strength and fairly high tensile and bending strength. Most aquariums have a plastic rim around the bottom and top. The weight of the water is distrubuted evenly across the bottom glass, the weight is then spread by force vectors to the plastic rim which exerts a downward force on the stand. The highest downward forces will always be at the 4 corners because of the convergence of the x-ordinate, y-abscissa and z-plane forces. There will also be spurious forces along the length of glass at the bottom but the natural moment of inertia of the wood support will easily overcome those forces. Additionally I would use gussets in the corners and use corrosion resistant fasteners like stainless steel or brass screws.
I conccur with what the others have said about leveling. Proceed with the utmost caution if using shims to level.
If I have time this weekend I'll work up a typical finite element analysis and post a graphical representation. If you could give me the tank dimensions and style of stand I can calculate down to a few ounces what the stand will hold before the wood fails from shear stress.
Don't be too impressed folks, this is what I do for a living and sometimes for fun. 8)
