I wanted to ask before I drill 1" or 1.5" (can't remember the size) holes at the top middle if it would cause the glass to bow and possibly break. My 55g is tall and narrow but I wanted to see before I drilled if it's going to cause it to crack and break. Thanks!!!
The farther under the waterline a hole is, the more stress there will be. People are able to drill all sorts of holes, seemingly without problems.
With diamond bits, it's pretty easy. I like to make a dam out of clay to hold a pool of water around the hole, and also use a wooden template for the diamond bit, clamped in place, so that the bit doesn't wander.