fixative vs fixing agent

fixative and fixing agent both are nouns.

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

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

fixative: a compound (such as ethanol or formaldehyde) that fixes tissues and cells for microscopic study
fixing agent: a chemical compound that sets or fixes something (as a dye or a photographic image)

Other hypernyms of fixative include fixer.

fixative (noun)	fixing agent (noun)
a varnish dissolved in alcohol and sprayed over pictures to prevent smudging	a chemical compound that sets or fixes something (as a dye or a photographic image)
a compound (such as ethanol or formaldehyde) that fixes tissues and cells for microscopic study