flatfish vs saltwater fish

flatfish and saltwater fish both are nouns.

Word	Noun	Adjective	Verb	Adverb
flatfish	Yes	No	No	No
saltwater fish	Yes	No	No	No

As nouns, saltwater fish is a hypernym of flatfish; that is, saltwater fish is a word with a broader meaning than flatfish:

flatfish: sweet lean whitish flesh of any of numerous thin-bodied fish; usually served as thin fillets
saltwater fish: flesh of fish from the sea used as food

flatfish (noun)	saltwater fish (noun)
any of several families of fishes having flattened bodies that swim along the sea floor on one side of the body with both eyes on the upper side	flesh of fish from the sea used as food
sweet lean whitish flesh of any of numerous thin-bodied fish; usually served as thin fillets