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")
A Halo fish was kept as a lab animal in a small aquarium in sick bay of the USS Enterprise. (TAS: "The Terratin Incident")
A makeshift 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)
Captain Jean-Luc Picard kept a fish named Livingston in a spherical aquarium in his ready room. (TNG: "Encounter at Farpoint")
An aquarium was part of Doctor Paul Stubbs guest quarters aboard the USS Enterprise-D. (TNG: "Evolution")
In early 2366, Counselor Deanna Troi kept a small aquarium in her quarters. (TNG: "The Survivors")