Earlier quoted context omitted.
Fatigue from what? It's a stationary object housed indoors, not a car or plane that goes through several climatical, temperature and environmental changes to cause fatigue. Unless they were subjecting the tank to draining and filling cycles often, it seems more like a design/build fault that was a ticking time bomb from the start.
The fact an object is stationary doesn't mean forces aren't continuously being applied... Same goes for bridges and every load bearing object.
A fish tank a relatively constant by comparison.