this is an old topic but I'll add my experience to it. I drilled a hole in a 10 gallon vertical for the same thing as a drainage. I used a 3/4" bulkhead which required a 1.5" hole. The bit i used was from glass-holes.com. I also drilled holes in the top for a mist nozzle and for wires of my temp / humidity probes to go in. I had to drill a 3/4" hole to allow the sensor of the exo terra temp / humidity to fit in. The mister's hole was a 5/8" hole.

Some quick things to make drilling glass easier

Use a template / guide hole or some kind of guide for drilling the hole to keep the bit from skipping around when you first start drilling

Don't push on the drill, let gravity and the weight of the drill do the work

Keep the bit cool by using plumber's putty and water or my favorite method a garden hose with constant water running over the drill bit and surface area of the glass

Take your time, glass drilling is a slow process to be done right as you are not drilling it like it was wood, you are actually grinding the glass away