The actual drilling is the easy part, if you set up correctly. A template jig clamped to the glass works really well. I've always used a template and scrap piece of glass on the back side. The template positions the drill precisely, and eliminates walking. It can be made from a scrap piece of plywood. The glass on the back side is not needed, but you'll get a cleaner hole with minimal chipping as you end the cut. Also, think about the glass hole that will drop, protect the tank glass so that as you end the cut, the glass disc does not drop on the opposite glass panel.
Measuring is critical. Be 100% sure you have the hole positioned correctly.
Know how to check for tempered glass. I wouldn't even spend time on planning to drill the glass, unless you've checked to make sure the glass is not tempered. You can use polarized sunglasses with a LCD screen, with MS paint on a white background. Tempered glass has stress marks or strain patterns that are clearly visible using this technique. The tank you have is probably not tempered given the size, but it is easy to check.
