Glass is actually very easy to work with, if you have the right (very inexpensive) tools and a little practice. All you need to cut glass is a good quality glass scribe, some cut run pliers and a straight edge. Mark it, scribe it and a squeeze of the pliers and you are done with the cut. It is seemingly a lost art, but some of us old reefers still prefer to work with glass.

To take the mystery out of it for others, here are a few pics I took when I was building my new sump.
The jaws of the cut run pliers.
Center the pliers over the center of the scribe and begin applying pressure until the cut runs to the other end of the piece.
The cut is quick and clean.
A quick pass with some sandpaper to smooth out the sharp edges and you are done.