fish tank vs storage tank

fish tank and storage tank both are nouns.

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

As nouns, storage tank is a hypernym of fish tank; that is, storage tank 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
storage tank: a large (usually metallic) vessel for holding gases or liquids

Other hypernyms of fish tank include tank, vivarium.

fish tank (noun)	storage tank (noun)
a tank or pool or bowl filled with water for keeping live fish and underwater animals	a large (usually metallic) vessel for holding gases or liquids