TLDR: Cause unknown, maybe related to freezing temps.
Germany has made an effort to reduce electricity usage, and I’d imagine this would require huge amounts.
Had they removed the fish and let it cool? I guess its cylindrical shape is dependent on it being full of water, so they could drain it.
I am not any sort of expert by the way. Perhaps I should put that caveat at the top of my reply.
Except
> I guess its cylindrical shape is dependent on it being full of water
no, or how do you mean that?