I recently added support with 2x4's to a 30 gallon aquarium stand that I bought used. I also added a piece of oak plywood on top of the stand and secured it with a nail gun. The problem is, it seems the plywood makes a slight hump in the middle so the aquarium isn't level on all 4 sides. It's a 30 gallon glass aquarium with black silicone and black plastic rimming on the top and bottom. Should I put some foam under the aquarium? If not, what should I do to level it out? Thanks.