drilling a tank

[Proteus]

Set the clutch on your drill. This will eliminate cracking from torque. Also slowly rock bit as not to drill level

[BlueWorldAquatic]

Make sure the glass ins't tempered, use a set of polarized glasses (fishing glasses) to see if it is.

slow and steady, with water for lubrication. Doest hurt having some wood clamped on the other side to reduce chipping also.