8mm glass is too thin, I would think. My old 70 gal
tank (standard height), with untempered sides, uses almost 1/2" thick glass ~=12mm. My new one is ~10mm.
The actual thickness you need is determined by the height of the tank, the taller tanks need thicker glass, even with less water in it.
This is the tank calculator:
You can use tempered glass to reduce the thickness, but that needed to be cut first before tempering, so is really feasible in factories. Plus, you will not be able to cut or drill it at all after the tempering.
Other possibilities would include - the tank not being square, putting extra stress on the glass; a deep scratch that had weaken the glass; or a chipped at the edge that propagated into a crack. <That happened to my old tank. I cut a u-shaped slot for an overflow, and I must have chipped a corner, as a crack propagated from the corner after several months. I should have drilled holes at the corners so the corner was rounded to prevent the stress ..... but that was an old scratched up tank I was playing with .....>