No, you can't drill some tanks. You can't drill a tank that has tempered glass. Many tanks have tempered bottoms, and there are some tanks that are all tempered.
You can order diamond bits for drilling standard (plate) glass tanks from glass-holes.com. They are, for the price, pretty good bits.
Smaller tanks--- 20g and under, can sometimes be drilled using a dremel if you have a STEADY hand, a steady stream of water, and a straight shank diamond bit ($9 at Lowes). I've drilled several tanks like that already.
There are all sorts of videos on youtube on drilling glass, they can show you better than I can explain.