You asked Evil1 about the weight of the canopy.
I have made several canopies over the years and have always used 3/4" plywood ( especially on 72" long tanks) and some of the canopies were fairly heavy. The ones I made when I used metal halide lighting and VHO actinic had to be 14" tall. These required 2 people to lift them off. With the wood, fixtures and the ballasts, I'm guessing 75-80 lbs.
When sitting on the rim of the tank, all of the weight was pushing down on the vertical panes of the glass. They aren't going to bend and easily support the weight. The rim was 192" around.