Change one middle brace to be at an angle? Diagonal braces are good for another eason - perfectly rectangular braces are not very resistant to side forces (think some young kid running into it full blast and pushing it sideways at the bottom of the tank). That's not an issue if you plywood the sides of course.
Or just remove one side brace -- hard to tell from the small image but it looks plenty strong (though are the side braces resting on, or nailed into the corner posts -- nailed into is not great and I'd be leery of removing a side brace then).