Just drilled over 20 holes in my tank and sump without issue, you get a feel for it. I used the diamond bits available at Princess Auto for not a lot of money and they worked fine. They are only good for 10 or so holes each before they were worn.
I have a nice Dewalt cordless drill but it has built a lot of things over the years and I noticed it wasn't true enough to use on the glass. I just bought a $20 corded drill at PA to use because new meant it was straight.
I also found that having a wooden jig with the same size hole as the glass hole saw was the easiest way to start a hole and that plumbers putty worked well as a damn for the cooling/lubricating water.
