Thinking about drilling my tank. But have some ?'s first

We've used glass-holes overflows on several custom tanks for customers in the past. They make a great product. Just don't put it too low otherwise the water line will be visible below the top trim of the tank which doesn't look good in my opinion. You may want to get some ear plugs as well... those diamond tip bits on glass make a very unpleasant sound. It's relatively easy to do, just very time consuming. Should take 3-5 minutes to drill one hole. It's not a typical drill bit that goes straight through, it just gently rubs away the top layer of glass until you're all the way through. It you try pushing through, you'll crack the glass... just take you're time. You absolutely need to keep it wet and cool as well. If the drill bit starts creating friction on the glass and getting hot, it will likely crack.