In the manufacture of custom aquariums while in college, I learned quite a bit about handling glass. But let’s be clear, I am merely a rank amateur. The guys in the back office at glass shops know more about glass that you even know there is to know. I don’t. But I do know how to fit plate glass with tubing for water, air, CO2, and nutrient solutions and that means holes.
I have occasionally had to drill small holes (up to 3/8” or 10mm) and for these I used blunt-nose diamond drill bits on my normal drills (a Craftsman corded drill or a DeWalt 14.4V cordless) at high speeds. By high speed, I mean that you can adjust your drill up to 10,000 rpm, not that the holes are created quickly. The blunt-nose bits are coated along the face and up the side with little diamonds that grind away at the glass. This takes time. The last thing in the world you want to do is rush the process. I have ruined projects this way. Be slow. You want to let the hole determine the speed and just hang out until the end.
You also want to keep the hole constantly wet. Not moist, wet. The water that you’re applying reduces friction, carries away heat from the friction that is inevitable, and washes away the slurry of glass powder. I mostly used a pitcher or glass to apply water, but that requires a hand -- which is bad if you’re using a hand drill. One place where I lived we had a lapidary shop set up which included small water lines that were easy to move around that provided a steady drip onto the grinding wheels. I was able to adapt those for my glass drilling. If you can rig up a steady drip, I suggest that for hands-free operation.
Also, I found a fair amount of chipping happened around the outer edges, particularly on the back side of the pane, which made for messy and really sharp edges. I got in the habit of doing two things to avoid this: slow down as you’re getting near the opposite side -- I mean really slow, and put masking tape on both sides of the glass where you’re drilling. The tape increases aggravation since you can’t see when you’re about to break through (though you can also remove the front piece once the hole is started) but it seems to reduce the chipping and in any case it collects what chips do occur. Another thing you can do to reduce the chipping, at least when you can be sure of getting the right spot, is to switch sides half-way through the hole. If you grind through from one side, and then finish from the other side of the glass, you don’t get that same kind of surface tension as the hole is finishing up. But don’t use that as an excuse to go fast.
Those professional glass workers that I mentioned above go way faster than I do after my meager 70ish holes of experience, but even they don’t go fast. They don’t want to pay for two sheets of glass when they can only charge for one.
If you want a hole larger than 3/8” then you need to use a hollow hole-drilling bit (though you can also get them at least as small as 5/64”). Like the blunt bit, the circular bits are diamond coated. Getting started with these is a little trickier than with the blunt bits, so I almost always arranged to use my drill press. When that was not an option (like when working on a previously assembled tank) then I’d use a pilot hole through a piece of scrap lumber clamped to the glass. But a drill press is really the way to go. The other advantage of using my drill press was that I could adjust the overhead drive belts to get exactly the speed I wanted. Unlike when working with the blunt bits, using a core drill is a process that requires you to gear the drill down to about 400 rpm. The larger the core bit you’re using, the slower you need to go since the speed at the diameter increases with the diameter. I have never drilled with anything larger than 3/4-inch so I can’t give personal experience, but I know that you have to gear down to 100 rpm for a three-inch hole.
As described above, go slow. No, really. You don’t want to overheat your fairly expensive diamond core bit and you don’t want to crack your sheet of glass. Boring a 3/4 inch hole through 1/4 plate glass can take you over ten minutes. Using the press, it is easy to apply too much force. Don’t. Allow your hand to rest gently on one of the lever-arms so that the end of the bit is in contact with the glass, but don’t put any real weight or pressure on. You need to give the grinding process time.
You also want to keep these holes wet. You can use the same kinds of processes for lubricating the hole as I described above, but another trick is to use clay (any kind of putty or plasticine would work, but as a potter, I always had clay around) to build up a small dam around the hole-site and just fill it with water. At the speeds that you’ll be drilling, you’re not going to messily shoot the water all over the place and with a pool of it, you can be assured that there is enough. As the water gets cloudy, just wash it away from a fresh source of water and keep going.
Always remember that when working with a drill press, you want your material to be securely fastened to the press plate or bench so that it cannot move relative to the press. Also, with glass, more than most materials, you need it to be pretty much perfectly level. If it flexes more than just a smidge it will break. This isn’t a problem for a smallish sheet of fairly thick glass, but my experience was that the only small sheets I used were pretty thin and the only thick glass I used was in pretty big sheets. You’ll want several points of support at precisely the right height around the press-shelf if the glass extends out very far at all.
You can also find a variety of sizes of V-tipped carbide bits that are supposed to cut glass, but my attempts and research suggest that this is a poor way to go. Stick with diamond grinders for good results.
As a note, most commercial aquariums are built from tempered glass. You can’t drill it. I mean, except that sometimes you can. It drills poorly and cracks easily. When I’ve done it, I used a deep guide, ~1/2-inch wood with a hole to guide the grinding bit and just took an extraordinary amount of time. And sometimes I ended up replacing one face of the tank because it broke.