If the tank base frame sits on the 2x4's then how are the 2x4's attached to the 4x4's? The screws or bolts that join the 2x4's and 4x4's will carry all the weight. You need bracing between the top and bottom 2x4 frames.
You can see how I remedied the same problem on my 1st stand. Kinda overkill but it holds well.