With the way it broke I suspect frame support could be involved. I find it interesting that the triangle break pattern is similar in size and distance from each end, this might imply bending/bowing on that side. If it were a glass flaw or impact damage I would expect a single point of origin.
Did the long rail under that side of the tank bow up or down causing uneven pressure on the frame?
Did the long rail under that side of the tank bow up or down causing uneven pressure on the frame?