I just did the deed. I drilled a 25 high and wanted the gasket for the bulkhead on the inside, when I got near the end of drilling, just before the circle of glass broke through, I some glass flake off. I can't install the bulkhead gasket on the inside as I want, I had to install it on the outside.
I suggest drill on the side of the tank where you want the gasket to be installed. This way you are gauranteed not to have chips on the side of the glass with the bulkhead.
I couldn't fit my drill with the diamond bit on the inside of a 12" tank.
Very easy to do. I used a cordless and it works fine.
rich