Plywood tank question

[joeytoe]

I am thinking of using the furniture grade plywood only because it is smooth, least amount of knot holes and within my budget.

Id go with Sanded plywood as its got a smoother surface, probally the same as furniture grade.
Cabinet grade plywood would be more costly an not worth the extra $$$$$$ as youd be covering the outer layer with fiberglass an or epoxy.