Building an Aquarium Stand
Properties of Commercially Important Woods
Saltwater.com post from Structural Engineer (Bridge Man)
"According to ASD Structural Lumber Supplement, a No. 2 southern pine 2x4 in bending can handle ~4590 lb-in of moment.
Assuming an even distribution of load across the 48"- 2x4, this equates to ~16 pounds per inch.
A 90 gallon, of 20" tall x 18" depth gives ~13 pounds per inch.
You would have two 2x4's running the length of the tank (one in front & one in back), therefore each would only see 6.5 pounds per inch, which is about 40% of their capacity. Plus, you have to remember, that the vertical pains of glass really aren't bending vertically along the length of the tank. This means that the horizontal 2x4's really aren't seeing an evenly distributed load across their length, but rather the load is concentrated on the ends, where the corner posts are located.
Therefore, 2x4 horizontal members are adequate.
For corner posts, a No. 2 southern pine 2x4 at 4' long can carry 7080 lbs, while a 6' long post can handle 4780 lbs. These vertical member values are for wall studs, which are braced with plywood against bending about their weak axis. As you can see, one 2x4 could hold the weight of the tank, but obviously four will be used." As someone already mentioned, beef up the connections, and you should be fine. A tank held up with 2x4's doesn't look safe, eventhough it may be. Your best bet is to build a stand that looks like it will hold, that way you will have some personnal reassurance that nothing will fail. And be sure to brace the stand against swaying front to back and along the length of the tank. Hope this helped, Bridge Man