The Radisson Blu in Berlin's lobby's massive aquarium, which was holding a million liters of water, has broken, flooding the establishment and the streets around it.
The "AquaDom" is 15.85m high (52 ft) and is said to be the largest free-standing circular aquarium in the world. It is home to 1,500 tropical fish.
After the explosion, glass that fell on two people caused injuries.
Police described the damage as being "amazing." A video showed water flowing into the hotel lobby from an empty tank.
As a result of the incident, guests were escorted from the hotel around 05:50. (04:50 GMT). The vast majority of the fish had died, according to a spokesman for Berlin's fire brigade, who also told that the chilly weather had made rescue efforts more challenging. More than 100 distinct species were housed in the tank.
In front of the Radisson Blu's front doors, which are now hanging into the street after being bent at an angle by the force of the tank explosion, was a mass of debris. With his girlfriend, Paul Maletzki was lodging on the fourth floor.
He recalled shaking and hearing a loud noise that awakened him. He noticed water streaming across the lobby when he peered down into it. Later, armed police officers led them and other visitors out of the hotel.
Franziska Giffey, the mayor of Berlin, visited the hotel to assess the damage and compared the tank explosion to a tsunami.
She expressed relief that it had occurred so early in the day, noting that if it had occurred an hour or so later, the lobby and the street outside would have been crowded with guests, many of whom would have been children. In an interview with local media, Sandra Weeser, a member of the German federal parliament who was staying at the hotel, said she had been awakened by "a kind of shock wave" and that the scene outside the hotel was "a picture of desolation." She claimed that potentially salvageable fish had perished due to freezing, and she recalled seeing a "big parrotfish laying on the ground, frozen."
Since the aquarium was updated two years ago, visitors can use a clear-walled lift inside. The hotel advertises that some of its rooms have views of it.
More than 100 firefighters were dispatched to the location, according to Berlin's fire department, and the cause of the break was not immediately known.It claimed to have utilized rescue dogs to search the hotel's interior for any potential victims, but no one was discovered.
Police warned locals to drive carefully as "huge amounts" of water were gushing into the streets nearby.