Don't make me go back there! Please!!! I pulled this picture off RC over ten years ago. I don't like it over there! :( I will give you the Reader's Digest version, because, at the time, it had already been broken into two threads because of the length.
For tanks up to four feet, all dimensional lumber are 2 x 4's. Over four feet, the top frame needs to be 2 x 6. No center support is needed. You do need to "skin" the stand to keep it from racking. I'd highly suggest also using a 3/4 inch piece of plywood on the top to "level" or smooth out the dimensional lumber on the top so the tank sits flush to the top. Here's the frame I build for my five foot tank in 2006/2007 using this plan:
All finished, it now looks like this:
That being said, my latest builds for smaller tanks (40B included) has been with paneled sides. This is the build I'm working on right now for a 35 cube. Uprights are 1 x 3's with 5 mm plywood panels.
EDIT TO ADD:
I'm an idiot.....I went back there and found the two posts. Note that because of Photobucket attempting to extort past users, many of the pictures are no longer available.....Including the one I posted here showing the construction plan. But here are the links to the two "split" posts over on RC:
Original Post - DIY Stands Template and Calculator
Second Split Post
Happy reading!