storage tank vs vessel

storage tank and vessel both are nouns.

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

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

storage tank: a large (usually metallic) vessel for holding gases or liquids
vessel: an object used as a container (especially for liquids)

storage tank (noun)	vessel (noun)
a large (usually metallic) vessel for holding gases or liquids	a craft designed for water transportation
	an object used as a container (especially for liquids)
	a tube in which a body fluid circulates