The “AquaDom” – home to 1,500 tropical fish – is 14m high (46 ft) and was described as the largest free-standing cylindrical aquarium in the world.
Two people were injured by glass.
Police said there had been “incredible” damage. Video showed an empty tank, water pouring into the lobby and debris strewn everywhere.
Guests have been moved out of the hotel following the incident at 05:50 (04:50 GMT).
Outside the Radisson Blu, a pile of debris lies in front of what were the front doors, which now hang into the street – twisted at an angle by the force of the exploding tank.
Paul Maletzki was staying on the fourth floor with his girlfriend.
He described being woken up by a loud bang and shaking. When he looked down into the lobby, he saw water flowing across it. They and other guests were later escorted out of the hotel by armed police.
The Mayor of Berlin Franziska Giffey went to the hotel to see the damage and described the tank burst as being like a tsunami.
She expressed relief it had happened so early in the morning – saying an hour or so later, and the lobby and street outside would have been busy with visitors, many of them children.
The aquarium was modernised two years ago, and there is a clear-walled lift built inside for use by visitors. Some of the rooms in the hotel are advertised as having views of it.
Berlin’s fire brigade said more than 100 firefighters were in attendance and it was not clear what caused the break.
It said it used rescue dogs to check the area inside the hotel for anyone potentially injured, with nobody found.
Police said “massive amounts” of water was flowing into nearby streets and people in the area should drive cautiously.
A police source told local media there is no evidence the break was the result of a targeted attack.
Berlin’s public transport authority said Karl-Liebknecht street outside the hotel had been closed off due to “an extreme amount of water on the road”. Trams service in the area was also stopped.
The aquarium contained more than 100 different species of fish.
On its website on Friday it said the attraction was temporarily closed and asked visitors to reschedule their tickets.
AquaDom was opened in December 2003 and was given the Guinness World Record for being the world’s largest cylindrical aquarium.
According to reports at the time of its construction, it cost about €12.8m (£11.2m) to build.
- Clarification: An earlier version said the aquarium was operated by SeaLife. Although the aquarium is in the same building complex as Berlin SeaLife, they are not responsible for the AquaDom
Source: The BBC