fish tank vs vivarium

fish tank and vivarium both are nouns.

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

As nouns, vivarium is a hypernym of fish tank; that is, vivarium is a word with a broader meaning than fish tank:

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

Other hypernyms of fish tank include storage tank, tank.

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