tuna vs tuna fish

tuna and tuna fish both are nouns.

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

As nouns, tuna and tuna fish are synonyms defined as:

tuna and tuna fish: important warm-water fatty fish of the genus Thunnus of the family Scombridae; usually served as steaks

Other synonyms of tuna include tunny.

tuna (noun)	tuna fish (noun)
any very large marine food and game fish of the genus Thunnus; related to mackerel; chiefly of warm waters	important warm-water fatty fish of the genus Thunnus of the family Scombridae; usually served as steaks
New Zealand eel
important warm-water fatty fish of the genus Thunnus of the family Scombridae; usually served as steaks
tropical American prickly pear of Jamaica