Drilling glass tanks is INCREDIBLY easy
After getting a quote of $100 from the glass store to drill three 1.5" holes in my 50 gallon tank with no guarantee that they wouldn't crack it, I decided to do it myself.
I bought a 1.5" diamond hole saw off eBay for $13 + $3 shipping.
To prevent the bit from 'walking' during the initial drilling, I made an 'L' out of two pieces of poplar I had laying around. The poplar 'L' was placed on the inside bottom of the tank with one side against the back glass. The bit was positioned inside the corner of the 'L' so that when it turned it would want to walk into the poplar 'L', which being held by my own hand ensured the bit wouldn't walk. This seemed fairly successful.
The cutting was very easy, the only part you have to keep an eye on is keeping the bit square to the glass. The more of an angle you have, the more chip out you will have on the back of the glass. For this reason, drill from the INSIDE TO THE OUTSIDE. Bulkhead gaskets go on the inside of the tank, so you want the best edge there with no chipout. The backside is less important, so a little chip out is ok there.
Drill anywhere from 60 to 180 RPM, just by sight. Put a marking or something on one side of the bit so you can gauge how many times per second it is going around. 1-3 times per second is OK.
During the cutting, keep a stream of water from a garden hose flowing over the area to cool the bit and wash away glass dust. Remember, you are sanding - NOT cutting - the glass away.
When you get towards the end, speed the drill up and do not apply ANY pressure. This will also help reduce back side chip-out.
I have never, ever in my life drilled glass before. Not even tile, or anything remotely similar. Today, using the method above, I put three 1.5" holes in the bottom of a 50 gallon tank successfully and for 16% of the cost a glass shop would have charged me. It was very easy, took about an hour to an hour and a half to drill three holes, and looks pretty good IMO.