I would add a section of 2x4 across the top joining the rear and the front sections as you have done to the bottom, this will make sure it is a stable. but yes, you could put a lot of weight on that thing. People under estimate the straight of wood. As long as you are compressing it through its length, a single 2x4 is quite strong. Just use common sense. It's a relatively narrow cabinet so don't go crazy high with the tank.