Drilling Glass
The Right Tools

Obviously to drill a 1 1/2 inch or larger hole in glass you require the right drill bit.  There are basically two methods to drill glass.  The quickest way although more expensive is to use a diamond bit, of which there are two basic types.  The lower cost method although more time consuming is to use a steel bit with the addition of an abrasive grit.  I used a diamond bit and only after wearing out two of the cheaper types did I discover a much longer lasting bit. 

 Diamond bits come in two types.  One has the diamond coating fused to the outside of a metal bit, and only lasts for about 25 to 30 pieces of1/8 to 3/16 inch glass.  I was able to drill only about 30 29 gallon tanks with each $80 bit :)  The better bit has a diamond and metal matrix fused to the end of the cutting cylinder.  This construction is very similar to a standard tile or brick cutting saw blade.  The one I use is model number PD 112 from C.R.Laurence Co.  They claim it will cut 1000 to 2000 holes in 1/4 inch glass.  I paid about $130 several years ago.

Drilling Through Glass
A hand drill and a steady hand can work.  Just be aware that any tilting of the bit while it is halfway through the plate glass may cause enough stress to crack it.  A Drill press or drill guide is a much better way to drill through glass.  This keeps the bit at a 90 degree angle to the work and helps prevent glass breakage.  These bits also need to be used in a bath of water to keep the bit cool and allow the fine particles of glass to be washed away.  If drilling on the inside of the tank, especially the bottom, a 3/8 inch layer of water can be placed into the tank while drilling.  Just be sure to keep the end of the cord out of the water :)  If drilling from the outside of the tank take it outside and run a slow trickle of water from a garden hose over the cutting edge of the bit.  I use a general products #36 drill guide, and have only cracked one tank out of over 100.  Use light pressure especially at the end of the cut to keep from chipping out too much glass at the end.  Have the underside supported as well.  This also prevents too much chipping.  On large tanks I make a shallow cut on one side of the glass and finish it up form the other.  This gives the best final appearance and avoids chipping.  Just make sure you line up the drill from the 2 sides.  A final sanding with emery cloth will get rid of the sharp edges.