Lunchbucket,
2X8 Upper frame and 2X4 for the rest is just fine strength wise.
I think this is more along the lines of what you described. This would allow for wires to be run up the back and hidden by a 1X4 board running up each side. The pink board in the top frame sits inside the ends. In the bottom frame you could install two, one under the legs and one at the very back of the stand, that would allow you to install a 18" deep sump. My 75g which is considered 48" wide by 18" deep is in fact 48.5" wide by 18.5" front to back. As such, I would guess that a 125 would be 72.5" long by 18.5" wide. If i would building for that size tank I would consider a 73" by 19" place for the tank and then whatever you want behind it. When I did the layout on my own stand, I made it 36" tall so the tank is closer to eye level but short enough so I can work in it with just a step stool.
