One recommendation I have is to take some foam ( or even wood ) and drill a hole through a small piece of the foam. Tape the foam to the glass with the hole in the foam lined up with where you want the hole in the glass. Also tape a piece of foam on the back ( no hole necessary with that one ) The hole you just drilled through the foam will act as a guide and keep the drill straight. The foam on the back will stop you from punching through the glass too fast and cracking the panel. All it takes from there is a slow trickle of water into the hole in the foam and you're set. Just take it slower than you would drilling a hole through wood. Works a charm! Diamond hole saws can be found online for less than $10. So with the money you save you can drill quite a few holes!

If you're not comfortable drilling your own hole, find a hobbyist who is willing to help, or visit any local fish store. Most if not all of them will drill a hole for you. Prices may vary of course, but usually they're reasonable.

 

I drilled three 60mm holes in the 300 gal. tank I had.  The glass thickness was about 11/16". 

 

In the limited experience I have, it seems that the thicker the glass the easier it is to drill.

 

Go to ebay, buy some really cheap diamond core bits and drill your own tanks. 

 

Adam