Nice job! Very clean build! But...
I would not trust it. You will get sag in the wood over that large of a span and your corners are weak, that is where most of the pressure is. I would put a 2x4 rib around the 2x12 as a "fix", with the 2" part facing the the 2x12 and the 4" facing up, or get some steel "L" brackets to hold those corners together. Did you use screws or nails for the corners?
I say this, because from experience, a 2 x 12 is strong in one direction, not the other, it will definitely bow over time, maybe not enough to blow the tank, but it would make me nervous. I would build the walls out of 2x4 and plywood, 16" on center just like a normal wall.