Drilling the Overflow Holes:
This was not as easy as it sounds. I tried calling around to various local glass shops and they all told me the same thing. They would not drill my aquarium for me with the glass in place. They would attempt it for me only if I removed the pane of glass myself prior to them drilling. I don't know if anyone has tried to remove the plastic frame from a tank without damaging anything, but I found this to be an impossible task. It's just glued together too well and in cracks that are too small to get into with a blade to cut it loose.
I desided to try drilling it myself. I read up on how to drill glass. Here are some links to sites I found helpful.
The hard part was finding a glass drill bit that was large enough to do the job, and still be cheap. I wasn't going to pay $60-$70 for a drill bit for just 2 holes when I wasn't sure it was going to work. Home Depot stocks glass & tile drill bits, but only up to 1/2". I needed 1.5" (40mm) holes for my 3/4" inside diameter bulk head fittings. I finally found a 40mm drill bit on Ebay from an online store in Hong Kong for about $10. I know probably low quality tools, but I only needed it to work for 2 holes (and it did).
I also bought a cheap $10 10gal aquarium to practice on, before spending $40 for the 20gal high tank I intended to use, figuring if it didn't work I wasn't out much. After having no problems with the 10gal tank I went and got the 20gal tank I wanted. When buying a tank to drill make certain that it is not made of tempered glass or at least not on the side you intend to drill. From what I have read tempered glass will shatter if you attempt to drill it. Some tanks only have tempered glass for the bottom pane and regular glass for the sides, while others are either made of all tempered or all regular glass. Most small tanks are all regular glass.
Drilling glass is very different from drilling wood. When drilling wood the drill bit cuts it's way through, where as with drilling glass the drill bit grinds it's way through like sand paper. When drilling glass you don't want to apply much pressure to the bit just enough to hold it in contact with the glass. You also do not want to generate any heat while drilling that might crack the glass. To avoid heat while drilling I used water as a lubricant between the bit and the glass. To keep the puddle of water in place I used some of my daughter's play dough to mold a ring around it, then filled the ring with water. Drilling glass also takes much more time than drilling other things. For example it took me about 30 minutes to grind my way through the 1/8" thick glass on the 10gal tank, and about 45 minutes per hole on the slightly thicker glass on the 20gal tank.
On the internet I saw some examples of people using a hand drill, however I thought I'd get a better hole using my small bench top drill press. The problem with switching from a hand drill to a drill press is that you loose the feel for the amount of pressure you are applying and therefor have to be extremely careful not to apply too much force with the drill press.
I marked where I wanted my holes using a permanent marker, which comes off glass easily with a little rubbing alcohol.
For my holes I wanted them drilled in the back side pane near the top. I turned the top of my drill press so that the top and the table were over haning my work bench while the base was turned toward the work bench and clamped in place. I screwed a piece of wood to the drill press's metal table to protect the glass from being scratched. I mounted the tank on it's side with the drill press table inside the tank and against the inside surface of the side I was going to drill. I also supported the weight of the tank from underneath using some 2x4s I had laying around.
Here are some pictures I took during this portion of construction: