You need something to break up the space in the tank. Its too plain right now. Look at current tanks and push those to a concept tank or use them as reference. It needs to look like it was actually constructed. Because right now it looks too much like a box. I would expose some of the tread to break up space, push the canon some more to show the viewer how it works. Also, add some paneling or some ways to enter into the tank. I notice the hatch on top but I dont know how it opens up. Add a hinge, or hint at a way that it opens. Also a latter to get to that opening would help with the design.