Opinions on woodworking on stands?

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I think horizontal grain would look unusual. Put the seam in the dead middle, or deliberately cut the sheets into smaller pieces and create a number of vertical seams (maybe 3). I'm no woodworker, but I'm sure you can get the seams to look pretty good.

On another note, consider adding a catwalk around the back sides of the tank. These will give you something to stand on while you work on the tank. You'll have to lean all the way over the top of the tank in order to work on the front.