I used to work in a glass shop. the best trick is 1. use a sharp bit(obviously) 2. go (drill) very slow. 3.(some may not know this) use lots ofturpentine as a cutting fluid in the area being drilled. it help dissipate heat and reduces friction. with a good bit, it it should take about 10 minutes average to drill 1 hole. by the looks of the picture, the glass got too hot and had too much pressure. that sucks about your tank. sorry to hear it. best of luck with the next one.