"Tuna" vs. "Tuna fish"
The term "tuna fish" is used to distinguish canned tuna from the live animal, and originated when canned tuna first became popular in the early 20th century. Today, "tuna" is often used to refer to the fresh fish (as in a tuna steak or sushi) or simply the canned product, especially in casual conversation. The term "tuna fish" often specifically refers to the processed, canned product. When people refer to a fresh, seared steak at a restaurant, they usually just call it "tuna".
While technically redundant (a tuna is a type of fish), appending "fish" is a common linguistic phenomenon, similar to "chai tea" or "ATM machine", which can serve for emphasis or to make a short word less likely to be misheard.