Well I couldn't find my other dremel tool and diamond bits, so I grabbed a couple more sets of them (you can never have enough). So how can you drill a hole in glass without paying a glass shop anywhere from $20-30 per hole (most with no ganruntee on their work), or buying those expensive diamond holesaws that can run upwards of $100 each (and only cut one size hole) DIY - it! There are many ways and methods, here's what works for me.
DISCLAIMER: This is merely a guide to the method of drilling glass that has worked well for me, several times in fact, however the author nor CaptiveReefing can be held responsible for any damage, injury, or other mishaps (you are using an electrical appliance adjacent to water).
A favorite quote of mine; If you have a stupid head, the whole body may tend to sufffer.
What you'll need;
1) the tank or glass (duh) (NOTE: Tempered or Anealed glass will explode due to the surface tension created from the heating proceess)
DO NOT attempt to drill anything but regular glass
2) a sharpie or similar
3)a dremel tool or similar (Variable speed preferred here)
4) a set of Dremel 7150 bits ($5 at WallyWorld)
5)a place to work with access to running water, bathtub or kitchen sink works well for smaller projects, though the latter may not be your spouse's ideal location.
6) Saftey equipment, eye protection, gloves, whatever else you deem neccessary.
6) some time and patience
Lets get started! This example is on 1/4" glass.
1) Grab a Sharpie and mark out the hole where you want it, for bulkhead fittings I use the gasket as a guide.
2) The above set of bits comes with one round end bit and a long tapered bit, get the round ended one chucked up in your dremel, (note the collet size)
3) Get the water flowing, it's important to keep both the glass and the bit cool, and prevent glass dust or chips from flying every where. Just e trickle will do.
4) On the slowest speed possible for your tool (mine runs 5K-30K RPM) VERY lighting begin tracing the outside edge of the circle you marked, remember your bulkhead fitting has to fit through the hole. Use no pressure, just the weight of the tool is all it takes, we are grinding a hole, not driling. Take your time as this will provide a guide for the rest of the cut.
5) Once you have the initial area scored continue on low speed with the round bit and water, you'll make a LOT of passes round and round and round. eventually ending up with a deep channel similar to this. Keep going, you can turn up the speed a bit if you want to remove more material faster, but run the risk of cracking the glass from vibrations or a slip.
6) Eventually you will 'grind through', STOP.
7) Time to switch bits over to the long tapered bit to finish up. Start from where you broke through, again keeping the bit vertical grind out the rest of the material (low speed is best), follow the channel all the way around until the center drops out. You may use the same bit to clean up any rough spots, gentle pressure, no sense hosing up your work now to start over.
Test fit your bulkhead, adjust your hole size if needed.
9) That's it install your bulkhead and you are good to go
10) Before you pat yourself on the back, if you used the kitchen sink quickly clean up the evidence before you get caught. But be sure to clean up any glass filings and such that were left behind.