Porthos being super-hydrated in a tank
Two Xindi-Aquatics in a tank
A Hirogen tank
A tank was a large container of liquid.
In 2152, Doctor Phlox and Captain Jonathan Archer used a tank to super-hydrate Archer's pet dog Porthos, while the dog was undergoing surgery to replace his pituitary gland. (ENT: "A Night in Sickbay") Phlox also kept a Lyssarian Desert Larvae in a tank inside sickbay aboard Enterprise NX-01. (ENT: "Similitude")
In the Xindi Council chamber on the Xindi Council planet, the Xindi-Aquatics were contained in a large tank. (ENT: "The Xindi", et al.)
The designation of these receptacles – both those used by Phlox and the Xindi container – as a "tank" is taken from the episodes' final draft scripts. The tank in "Similitude" was scripted to be "a small Plexiglass container" which could be easily held, though the actual tank that appears in the final version of that episode is somewhat larger than described in the script.
The concept of using computer-generated tanks to contain the all-CG Xindi-Aquatics came about due to the difficulty of creating CG water. The tanks meant the visual effects artists didn't have to go to the trouble of creating CG waves and since the entire environment was digitally generated, it could be kept under control. The creation of the tanks containing the Aquatics was thus relatively very easy. "It's just like putting them in a smoke field," noted Dan Curry. The CG tank in the Xindi Council chamber was built by CGI company Eden FX. (Star Trek: The Official Starships Collection, issue 65, p. 11)
The Hirogen had tanks in their trophy rooms aboard their warships. According to tricorder readings by Commander Chakotay in 2374, these tanks included some sort of enzyme which breaks down bones, muscle tissue, and meat from different species. Tuvok speculated it could be a method to denaturate their prey while Chakotay assumed it could be their dinner. (VOY: "Hunters", "Prey")
The call sheet featured the describing note "plexiglass tank of meat".