As I said before I wouldnt do it with thin glass, like single or double strength (3/32 or 1/8"), but this is 1/4" plate. Glass is a bit like steel, thicker is actually more forgiving. Here is 2 videos of me drilling over an existing hole with a larger hole, I also had to drill another hole for the return line in the middle and had to do that at a VERY akward angle from the back, I didnt take a video because I had my hands full.
That makes 4 holes I have drilled in this tank so far without an issue. Its really not hard as long as you go slow, keep water flowing over the bit, dont push hard, and keep the drill square the the glass. I find it also helps to work the drill around in a circle a bit, that augers out the hole a little and there is less chance of binding, avoiding binding is also why I turn the clutch all the way down.
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