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.
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. Guests have been moved out of the hotel following the incident at 05:50 (04:50 GMT).
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.