I think I've seen it done once or twice on rc. One used a dremel and the other I can't remember. I've drilled glass tanks before and I'm pretty confident in my ability to do it without much problem. That being said, I wouldn't even consider doing it.
Are you 100% positive the glass isn't tempered?
How would you keep the drill bit cool and lubricated?
Then there is the "what if" something goes wrong? It's one thing to goof up and owe you a new piece of glass but a tank full of livestock, nope not me.
Not sure what size tank you have but it would be worth it to either brake it down, drill it and put everything back in. Another option is to upgrade to a bigger tank and drill it or do whatever a d transfer to the new tank.
If you do decide to try it... take a video cause I gotta see this