Here are some pictures of the construction of my 75 gallon aquarium stand. I used 2x6 lumber for the main frame and 2x4 for the supporting frame.
You can see the 2x4s that make up the inside of the leg on a 2x6 and then the second 2x6 is attached to complete the leg.
This is the leg free standing, the 2x4s are longer than the 2x6s so that the 2x6 aquarium footprint can attach to the legs.
My original plan for the footprint didn't include miter joints but it seems to have worked well enough. Usually my miter joints aren't perfect, the sides usually are not flush with each resulting in uneven corners.
The stand all framed together
Here's what an open corner looks like before the short end 2x6 was attached.
And you can see that the corners are not perfectly flush. I think such a small amount wont matter because the plastic rim of the tank should be taller than it but just in case I used a flush cutting / reversing backsaw to remove some of the extra height of the 2x4s.