The aquarium in the Enterprise's sick bay
An aquarium is a tank filled with water designed to hold species of life which require such an environment.
Kirk and Spock were held in a aquarium in sick bay after a forced mutation restructured their lungs to work under water. (TAS: "The Ambergris Element")
Captain Jean-Luc Picard kept a fish named Livingston in a spherical aquarium in his ready room. (TNG: "Encounter at Farpoint")
A make-shift aquarium was created on a captured Klingon Bird of prey to transport George and Gracie to the 24th century. (Star Trek IV: The Voyage Home)