Common mistake I'm afraid... don't overtighten bulkheads.
I've broken a few glass panels too, thinking "just one more quarter turn."
Usually this is more of a problem with 1/4" glass or less. If you can break 1/2" glass then you're making the bulkhead WAY too tight!
Basically if "slightly beyond finger tight" does not give you a 100% seal then something is wrong. Things to watch out for:
- the seal for the bulkhead usually has a little "ridge" on it -- this should face the glass
- it should be on the inside of the tank
- maybe there is a particle or abrasion/burr on one of the surfaces preventing a seal.
Failing all that .. putting a little smear of silicone around the threading or around the bulkhead side sometimes is needed (i.e., bottom of the tank and there is static pressure on the seal and so on). But in general it should not be necessary to silicone a bulkhead (I've done it and usually do it for added peace of mind, but the theory is that it's not "necessary").
I once broke a tank that I was drilling that had livestock in it. We had lowered the waterline so to install an overflow box. I had to drill on the vertical which is always a challenge but I had done so many times before. The trick though is to really, really, really back off on the pressure towards the end and .. well I didn't back off enough. I punched through, fell a little forward and.. crack.
Since this was a live tank emptying it was not an option. But the good news was that since it was where an overflow box was going to be installed, we were able to patch it up with some scrap glass, siliconed in place. And since it was an inwall tank the fact that it looked like butt didn't matter since it was a hidden surface. Anyhow moral of my little tangential story is that it IS possible to patch over a crack. In fact I have at least two patches in my sump for my 75g (again, overtightening bulkheads). An inconvenience for the time that the silicone is curing but only for a day or two.
It's of course better to NOT break the glass in the first place, but in general it is possible to fix these sorts of things. Just recognize though that it's not pretty.
