Howdy,
Is your question regarding glass or acrylic. Glass is a ***** to drill, while acrylic is a snap.
Glass is hard enough to drill that most places that do it, won't warranty the tank. If they crack the tank while they are drilling, you're screwed.
Now, acrylic is very easy and can be done several ways. I use a router with a 4 fluted straight bit to cut large holes in my tanks. For smaller holes, say less than 2", I use regualar carbide hole-saws hooked upto my dewalt drill.
The key to acrylic is to go slow so you won't "snag" the material and cause chipping or overheating.
Good luck,
Scott.