I called the manufacture of the tank and he said to definitely fix the gap. So I took off the plywood, which seemed to have a bow, and it fix the problem a little. All three corners are now touching the 2x4 rim (without the plywood top) there is a tiny 1/16 of an inch gap in the front right corner.from the corner out it goes from a 16th of an inch to nothing in both directions for about 2 inches. I am thinking about shimming beneath the foam. Any thoughts?