Looking like this is going to be a cool build. Interested to see what you do here
I am currently doing a massive build with 76 20 gal tanks & over the 2 x 4's I'm using 1/2" 5 ply plywood, primed & painted & then 1/2" Styrofoam over that.
It looks like you have little room to work with the top of the tank so close to the shelf above it.