species vs fish species

species and fish species both are nouns.

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

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

Other hyponyms of species include bacteria species, endangered species, type species.

species (noun)	fish species (noun)
a specific kind of something	a species of fish
(biology) taxonomic group whose members can interbreed