I find thin glass tanks are harder to drill than those with thick glass. The only tanks I have cracked during drilling have been the small ones like 20 gals, tanks of 75 gal and over with thick glass are much easier. I have drilled at least 10 tanks, some of them standing, and half full.
I like to drill my wood template with a hole saw for wood, put a small hole above where the glass will be so water can flow onto the drill bit and glass, and then use a diamond coated hole bit for the glass.