I went exploring for my limits ... and found them.
Oh well, I'm slowly healing.
One workaround for the tempered bottoms is to drill out the back panel instead of the bottom. I had to do this for my FW tank because the tank has a tempered bottom. You can see details of the build (including the overflow) in this thread:
http://www.canreef.com/vbulletin/showthread.php?t=28505
The odds of breaking glass are non-zero, but, the trick is to go slow on the drill and very little pressure. Like about 1/4 speed of the drill, very little weight pushing the drill, and let the drill "sand" its way down through the glass. It takes a few minutes (1/2" glass takes forever
) and once you're close to pushing through you back way off on the speed and pressure and then you just pop through.
You need some cutting oil and a little reservoir thingy to hold the oil in place as you drill through it (I have this stuff too). It's easier to show than to describe, sorry if it doesn't make sense.
I have busted .. hmmm .. I'd say about 3 tanks drilling, but have drilled at least 40 holes. So .. yeah, like I said, non-zero but less than say 1 in 10 odds. Each time I did break a tank there was a reason (got impatient, or was trying to drill a non-empty tank, and so on). I was able to fix each tank (patch or replace the panel in question). If someone has some scrap glass, maybe the thing to do is do a trial run. But basically, once you've done it once, you realize it's not too bad.