vivarium vs fish tank

vivarium and fish tank both are nouns.

Word	Noun	Adjective	Verb	Adverb
vivarium	Yes	No	No	No
fish tank	Yes	No	No	No

As nouns, fish tank is a hyponym of vivarium; that is, fish tank is a word with a more specific, narrower meaning than vivarium:

vivarium: an indoor enclosure for keeping and raising living animals and plants and observing them under natural conditions
fish tank: a tank or pool or bowl filled with water for keeping live fish and underwater animals

Other hyponyms of vivarium include aquarium, marine museum, terrarium.

vivarium (noun)	fish tank (noun)
an indoor enclosure for keeping and raising living animals and plants and observing them under natural conditions	a tank or pool or bowl filled with water for keeping live fish and underwater animals