I will be honest I have drilled quite a few operating tanks, but I don't like to do it. Myself, I don't worry about the glass getting into the tank, since the grinding residue is so small it makes very fine sand for the tank. I drill from the inside to the outside, so if it does chip as the hole is finishing the chips do fall out of the tank.
It is pretty easy to do, as long as the glass is not tempered, just grind slowly, and keep the bit wet so it does not grab at all. Drop the water level as low as you can before drilling and make sure you practice drilling before attempting this so you don't end up cracking the tank. I myself have drilled I don't know at least 75 tanks total and have never cracked one when drilling. I have found some tempered glass that I was not expecting, and I have cracked one tank a few years ago, when I was enlarging a hole from 1.5 to 1.75 inches. Just make sure you go slow and hold the drill as steady as you can because if you tip the drill after the bit is most of the way through the glass you can cause it to bind and end up cracking or chipping the main glass.
Kim