hot-water tank vs storage tank

hot-water tank and storage tank both are nouns.

Word	Noun	Adjective	Verb	Adverb
hot-water tank	Yes	No	No	No
storage tank	Yes	No	No	No

As nouns, storage tank is a hypernym of hot-water tank; that is, storage tank is a word with a broader meaning than hot-water tank:

hot-water tank: a heater and storage tank to supply heated water
storage tank: a large (usually metallic) vessel for holding gases or liquids

Other hypernyms of hot-water tank include heater, warmer, tank.

hot-water tank (noun)	storage tank (noun)
a heater and storage tank to supply heated water	a large (usually metallic) vessel for holding gases or liquids