water tank vs storage tank

water tank and storage tank both are nouns.

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

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

water tank: a tank that holds the water used to flush a toilet
storage tank: a large (usually metallic) vessel for holding gases or liquids

Other hypernyms of water tank include tank.

water tank (noun)	storage tank (noun)
a tank that holds the water used to flush a toilet	a large (usually metallic) vessel for holding gases or liquids