If you decide to drill the tank, here are some tips.
IMO It's unlikely that the tank is made from tempered glass. Maybe the bottom, but I doubt it. Just to be sure though, here's a neat trick I learned to find out. If you or someone you know has a pair of high quality sunglasses, with polarized lenses they will help answer the question. Put them on and take a look at the panels. If they are tempered, you'll see it. You'll see the oven/heat marks in the glass itself. It looks kinda wavy, with a uniform cross hatch appearance. Ever seen that wavy appearance in a cars windshield? That's what your looking for.
If it's not tempered, here's a few tips.
You don't need to take the tank apart to drill it. Just lay the tank with the panel your gonna dill facing up. Get a quality diamond impregnated bit. Not just the steel kind with diamond paste. Duct tape all around the area to be drilled on both sides. Use a 50/50 antifreeze/water solution to lubricate/cool the bit and glass at all times. Use a generous amount of coolant, and most important, take your time. Let the bit cut slowly, and don't force it.
I don't know how thick the glass is, but obviously the thicker it is, the longer it takes to cut. It wouldn't hurt to buy a small piece of glass from the hardware store to make a few practice cuts.
HTH