Most tanks under 55gal. are not tempered. Even when tanks are tempered, it's only the bottom pane of glass (99% of the time), unless it says otherwise.
If you dont have a RotoZip, you can guy a universal chuck that's made by Dremel and then you can use the Rotozip bit on your Dremel.
Once you see the RotoZip tile bit, you'll see what we mean by the cutting area. The Dremel bit is just the tip but the RotoZip bit's cutting surface is much, much greater.