hypo vs fixing agent

hypo and fixing agent both are nouns.

Word	Noun	Adjective	Verb	Adverb
hypo	Yes	No	No	No
fixing agent	Yes	No	No	No

As nouns, fixing agent is a hypernym of hypo; that is, fixing agent is a word with a broader meaning than hypo:

hypo: a compound used as a fixing agent in photographic developing
fixing agent: a chemical compound that sets or fixes something (as a dye or a photographic image)

Other hypernyms of hypo include fixer.

hypo (noun)	fixing agent (noun)
a piston syringe that is fitted with a hypodermic needle for giving injections	a chemical compound that sets or fixes something (as a dye or a photographic image)
a compound used as a fixing agent in photographic developing