Hey, I just drilled a 10 gallon myself to practice for a 33 gallon I am going to drill in a few weeks. You will need a diamond core drill bit. I got mine from here:
<a href="http://www.shopsmartxpress.com/AmeriGls/W9.htm" target="_blank">http://www.shopsmartxpress.com/AmeriGls/W9.htm</a>
Works great! Just don't put much pressure on the drill. Let the drill do the work. I took mine in the driveway, ran water from the hose over the glass while I drilled. Not a gusher, but a small stream to keep glass and drill bit cool while drilling. A portable drill press comes in handy because otherwise getting the hole started is the hard part. The bit wants to run all around on the glass. This scars the surface of the glass. The portable drill press keeps the drill or the tank from moving around on you. It will make things easier and a cleaner hole.
Once I got the bit started in the glass, it took like 30 seconds to complete the hole. Also, I drilled the side glass not the bottom. They say it is hard to drill tempered glass and most tanks have tempered glass on the bottom for added strength. Good luck, and Happy Drilling!
P.S. If you have any other questions, post them here and I will respond as soon as possible.