How do you test the long-term durability of a tank or it's ability to withstand unusual events? Even if it doesn't break on filling as it did in this case, just because it is holding water doesn't mean it is safe and will continue to hold water for years to come.
Personally I'd rather rely on maths to calculate the required glass thickness. That tank with 9mm glass has a safety factor of 0.6, so the glass wasn't even strong enough to support the water even under ideal circumstances. For a 3' tall tank you'd be looking more at 19mm glass for a reasonable safety factor.
Who ever built that tank had no idea what they were doing (unless they only intended to half-fill it). You're lucky it was the bottom pane that broke during filling, and not the front panel while you were standing in front of it admiring how well it was holding water!
