The others told me 15mm is plenty thick and that's just how old tank's are.
this is true. I was one of those people (who has had thick glass braceless tanks last for 20 years before selling them)
the lack of braces likely had nothing to do with the tank exploding.
there are a few threads on this board with people with large braced tanks exploding just the same.
problem is, when tank fails catastrophically, you can't really say exactly why.
it could have been dinged in transit
it could have been a defect in the glass that weakened when the tank was drained (relieving all pressure) and then filled again (putting the pressure back on)
its impossible to say for sure.
I always buy new as well, just because I figure the risks of damage from being drained/filled several times or damaged being transported time and again might be minimized.
but this could all be BS. lots of people by used tanks and have no trouble as well.
again, no one can say for sure. its easy to point blame, but in reality, thats just a way for us to try and rationalize why it happened, when we really can't be certain.
so sorry
glad no one was hurt.