15mm with a euro brace and at least one brace in the center for added support. The euro brace should be at least 4" wide.
You could use 12mm all around but no matter what the bracing it'll bow a little which will be noticeable when you get real close to the tank or try taking closer photos.
To save weight or cost you could just use the 15mm on the front and back and use 12mm for everything else.
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