I had the same issue on my 240g. I drilled some 1.5in. holes and found out later I needed them to be 1.75in. I had a thread going on this. Most people said it can be done with a Dremel, but it will wear out the bits quickly. It also depends on the thickness of the glass as well. I went ahead and purchased a new diamond hole saw to do mine. I'd rather have the hole nice and smooth and perfectly round.