I cut overflows in the back of my tank and used a alpha saw with water cooling and a glass blade. Bought the saw and blade and would lone it if you were in Austin. If you are making 4 long cuts to holes from a hole saw and the glass is thick... don't know as a dremel or roto zip will get er done. First you need to keep the cutting tool wet, then blade size, rpm and so on are a factor in how fast it will go. Also, the hole you drill needs to be big enough to get your return pipe through, think I used 1" and returns just fit through. There are some pics of the slots in my tank build thread (360 gal build under acropoorer).
360 Gal 72x42x30 - 110 Gal Cryptic fuge & 50 Gal sump
60 Gal & 30 Gal Frag tanks
3x400W MH - 940 W T12 Actinic - 90 W LEDs
Euro-Reef CS8-3
Deltec 601s reactor
1HP chiller
4xTunze 6100 & 6105
300 lb LR, SPS, LPS, fish & some softies