gas holder vs storage tank

gas holder and storage tank both are nouns.

Word	Noun	Adjective	Verb	Adverb
gas holder	Yes	No	No	No
storage tank	Yes	No	No	No

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

gas holder: a large gas-tight spherical or cylindrical tank for holding gas to be used as fuel
storage tank: a large (usually metallic) vessel for holding gases or liquids

Other hypernyms of gas holder include tank.

gas holder (noun)	storage tank (noun)
a large gas-tight spherical or cylindrical tank for holding gas to be used as fuel	a large (usually metallic) vessel for holding gases or liquids