young vs young fish

young and young fish both are nouns.

Word	Noun	Adjective	Verb	Adverb
young	Yes	Yes	No	No
young fish	Yes	No	No	No

As nouns, young fish is a hyponym of young; that is, young fish is a word with a more specific, narrower meaning than young:

Other hyponyms of young include hatchling, orphan, young mammal, young bird, spat.

young (noun)	young fish (noun)
any immature animal	a fish that is young
young people collectively

young (adjective)	young fish (adjective)
(used of living things especially persons) in an early period of life or development or growth
(of crops) harvested at an early stage of development; before complete maturity
not tried or tested by experience
being in its early stage
suggestive of youth; vigorous and fresh