I don't know much about physics, but if what the tweet says is true (that the “glass transition temperature” is 108 degrees Celcius), hasn't the aquarium always been operating in a glassy state?

Apparently so, but I think the argument is that it rapidly becomes more prone to cracking as the temperature falls.

On the other hand, the chart seems to be showing that its tolerance for stress, prior to the strain/stress relationship inverting, increases with decreasing temperature.

The night before temperature drop to minus ten degrees celcius put the tropical fish outside their normal temperature range and reduced the tolerance for strain before fracture of the tank. This is an awesome example of Twitter explaining things from a horse's mouth better than your mediocre mediacorp outlet.

This tank was indoors, right? And it was full of water presumably maintained at a temperature tolerable by tropical fish. If the heater or its power supply failed, it might cool somewhat, but that the outside temperature was about -10°C seems to be something of a red herring.