Barrie is the man with this, but I have seen a guy drill all his tanks to plumb them all in, he drilled them all intact and just used a glass cutting hole saw from mitre10 went really slow with his drill and started on an angle and used water from a spray bottle as lube.. He didn't break any and would have around 20 tanks in his room all plumbed up, maybe he was just lucky or really good at it, who knows but I have never given it a go