OK, lets see if I can make this a little clearer. My Google SketchUp skills are not what they never were. Anyway, what I am trying to show you below is the frame I used to mount my glass. You are looking from what would be the inside of the tank. It is a 2x4 outer frame with a 2x3 inner frame. The glass was sized to sit on the inner frame with about 1/4" play around the edges. After you build the frame, everything that is red and yellow gets coated in the fiberglass resin. The amber color is just wood. I think I used 4-5 coats. Once the resin has dried I lightly sanded it and siliconed the glass to the area that is yellow. Once that silicone has dried I ran a bead of silicone around the outer edge of the glass. When all of that had cured, a painted the liquid rubber that is coating the entire inside of my tank right up to but not touching the silicone
Oh, an on the overflow pulling from the bottom...the only thing I can think of that might work would be to plumb a bulkhead fitting going to the outside of the tank. Then, run the pipe up and have it tie into the overflow. I am not sure what kind of draw it would have and what issues I would create when the power fails.