Be sure the glass you're cutting is annealed and not Tempered, I think most 20's will not be tempered, but it's always good to be sure.
A dremel with a diamond bit will cut through it with no worries about breaking the glass - the bit will break or bend before you break the glass - you basically "sand away" the glass hole edges to cut it out.
Most glass shops will drill a tank, I don't know any LFS's in the area that will (The Hidden Reef in Philly used to drill holes for $20/ a hole, but they had a fire and have not opened back up yet)
For about $30 you can get a Generic Dremel tool and the diamond bits - one that looks like a ball (for starting the hole) and another longer skinny one - you use this in a slicing motion, moving the dremel up and down slightly, and constantly keeping the bit wet with a slow steady stream of water or a spray bottle. Be sure to get a dremel with a power cord, and not batteries - the holes take about 1/2 an hour - depending on size.