Hello,
This is a good topic to bring up, you can actually run into some real big issues when drilling tanks even half full. The stress factors involved in drilling a tank when it has water in it is huge. You may get luck like the some hobbyist who drill tanks and have had no issues. However you may end up with a 29 gallon tank on the floor. You must fully understand the concept of drilling glass, and how sensitive and how quick one "spider" can cause the whole tank, or pane you are drilling to break. More or less why risk it, just drain it, drill it and fill it. Laziness will only bring you problems, maybe not now, but down the road another time. Don’t cut corners when it comes to the liability of the livestock in your tank and this hobby. At any rate good luck to you and drilling your tank, what ever path you choose. See it work out both ways personally!
