There are some photos of what it looked like here https://www.uniqhotels.com/radisson-blu-berlin

Can't imagine the fate of all the fish, what a tragedy.

I'm betting on 'someone threw some Ninja Rocks at it'[1]

[1]: https://en.m.wikipedia.org/wiki/Ninja_rocks

Heh, sadly those only work on tempered glass and this was acrylic.

It appears the bonded seams between acrylic sections failed.

Cracks in glass travel so fast a regular CCTV camera won't be able to see where the break first happened.

Unless you see something like one of 3 teens swings hand and they run away.

Off topic

How do they make such a huge cylinder?

Carefully.

And in sections which are later bonded together. Alas, it is at those seams where tanks like this tend to fail, and when they do, it's instantaneous.