Woohoo! Tank has a hole in it, and it's the right size.
I also totally cheated.
Couldn't find an 1&3/4" diamond hole saw ANYWHERE I looked, and so I gave up. Went to Best Tile and bought a carbide hole saw for $14.00. Practiced on about 4 pieces of scrap glass and it worked, so I drilled the tank with it. For anyone who's ever wondered, I can clearly see why carbide is not recommended for glass drilling, it was MUCH more aggressive and I had to go MUCH slower to avoid chips. I still ended up with a messier hole than I have experienced with diamond saws. It's done though, so now I'm off to Tap Plastics to get a black background and maybe an overflow box if I'm lucky in the scrap bin.
Also, I feel it's important to mention that the hole saw had smaller "chunks" of carbide than any of the others I saw today, and I looked at a lot. I feel that this is important for a clean cut without chipping the glass. All the glass specific saws I have seen have a VERY diamond grit, and the carbide ones tend to be "chunkier" which could be detrimental when cutting glass. Aggressive bits equal a slower cut, because you have to be more careful.
Learning as I go but today has been fun.