Drilling a tank

[Kabong]

I would not reccomend using your current bi on that thick of glass.
Your going to need a lot of water flowing over the area to keep it cool.
So your abrasive compound would keep washing away.
If you dont mind waiting a bit, My best suggestion is to order a proper diamond hole saw bit.
You can get them pretty cheap off ebay
http://stores.ebay.com/THK-Diamond-Tools