Sorry, sounds like you have to get drilling!
It should be possible to drill a hole in a tank with livestock in it. I've had great luck enlarging and plumbing an overflow hole into the side of an active tank. The following instructions are basically a description of how I did that, altered for the task of drilling a hole
Lower the water level to about two inches below the level of your planned overflow spout. Then, use silicon to glue a 2" PVC elbow to the inside of the tank, over the spot you intend to drill. One end of the elbow should be flush against the glass of the tank; the other end points up. Cover the upward end of the elbow with plastic wrap and poke a hole in the wrap.
Now start drilling! Using an http://www.amazon.com/Wolfcraft-4525404-Attachment-4-Inch-8-Inch/dp/B000JCIMEA/ref=sr_1_1?ie=UTF8&qid=1409957324&sr=8-1&keywords=drill+guide, drill into the side of the tank while a friend pours water over the drill site to keep the glass and drill bit cool. The PVC elbow should keep glass dust out of your tank. As you start to poke through, your friend can start pouring water into the hole in the plastic wrap.
After you're done drilling, use a vacuum cleaner with a nozzle attachment to remove all glass dust from inside the elbow. Then, slide a razor against the silicone adhering the elbow to the glass. It should come off with a small application of force, and the remaining silicone can be easily cleaned up with the same razor.
And that's it! This strategy worked perfectly for me when I was enlarging my overflow hole; I don't see why it would be much different for drilling the hole from scratch.