[BlueAbyss]

If you're drilling the back, put the holes side by side so you can build an overflow box around them that doesn't take up a huge amount of space inside the tank. Of course, you don't want them too close together either. I'm not sure what a safe distance from each other is for the holes, but just make sure you support the glass well while you're drilling, don't put any pressure on it (let the weight of the drill do the work), and keep clean water flowing over it to clear the glass chips and cool the bit.