Like Oz said if tempered you can NOT drill, if your not sure if you have tempered glass or not then contact the company that made the tank and they can tell you. The bottom of most tanks are tempered with the sides and ends being regular glass.
I know for a fact that the 55 gal
tanks sold at walmart are tempered, learned the hard way while having it drilled.
Is it hard to do no, but it takes a steady hand a high quality diamond hole saw bit and lots of fluid for lubrication.
Guys in the reef club Im in drill tanks all the time with no issues, the club got a set of bits for use to use and it working out well.
The pros use a drill press, but with care and palnning you can do it free hand.