Originally Posted by Mark Hewitt
I wouldn't even want to do this while the tank is empty, as you'll have created a point of weakness in the glass so when it's refilled the whole thing may fail.
Ummmm.... not exactly the way physics work. People drill tanks all the time without issue. Several of ours are and have been fine for years.
The act of drilling is where the risk is, once it's done you are good. Doing it filled is asking for catastrophe though (IMO