I’m in camp “Midwestern American who says tuna fish”. . .but I’m also right there with the person that said they don’t order them and tuna fish sandwiches are something made at home.

For the record, I don’t know why the fish part is specified. It just always was. It’s not like my family called it a “can of tuna fish” growing up or anything. It’s just the sandwiches. Put that tuna between two slices of bread and suddenly the word “fish” gets thrown in there. Maybe it just sounds more fun if you add more syllables? Either that or somebody in the region had to explain that tuna was a kind of fish years and years ago and it just stuck.