If there is a complete frame of the 1x4s on the inside, the plywood will effectively just be cosmetic, with some reinforcement against racking.
Yup, I'm not worried about the plywood because it is only acting as a shear panel. It's equivalent to diagonal bracing, which mostly only adds strength against racking. I loved solid mechanics and structures in college; took some additional classes for my MS too. (hint... I know what I'm doing, so don't worry, folks)
Not shown in the original pics are the last couple jack studs that vertically support the frames under the tanks. Also, I'm gusseting (plywood triangles in the corners) the back of the stand for strength against racking in that plane. This is the only weakness that is preventing me from setting it up and getting some water flowing.
I don't have access to the tools to do better jointery right now. I just wanted to make something that was good enough and to show how most folks can get by with a few simple tools. Well... how to make something a little nicer looking than the utilitarian concrete block and framing lumber stand. (I'm not hating)