"Tuna fish" is a phase used primarily for canned tuna, but not for the live fish or things like tuna steak. It's because when canned tuna was created in the US in the early 1900's people who were not right next to the sea (like the majority of the US) did not know what "tuna" was. Firstly, the word is a of Spanish origin and secondly, its a salt water only fish. So in order to sell this to middle America, which was where most of the consumers were at the time but was also made up of people who have never seen the ocean, they added the word "fish" to show like other tinned fish that was commonly purchased: codfish, bluefish, and whitefish, this is also a fish and that is what you can expect when you open this can.

There's no one single reason, but the top theories:

1. Tuna oil was a thing before "tuna fish". Yes, people could have said "tuna" but they didn't. That's language for you. People say "ATM machine" and "PIN number", too.
2. "Tuna fish" has a slightly sing-song pattern to the stressed/unstressed syllables that probably contributed
3. For whatever reason, "tuna fish" tends to refer to canned tuna, whereas "tuna" can include fresh (or frozen) tuna.

It's… just how language evolves.

I think, however, that "tuna fish" is slowly dying out in favour of just "tuna". As a 50 year old, anecdotally I have seen the usage decrease in my lifetime.

I bought some tuna fish and chai tea with cash i got from the atm machine

Colloquially tuna fish refers to the shredded salt brined tins of fish like this:

Which I do think is worth distinguishing from the actual whole pieces of tuna

As an American who was only ever said tuna or tuna sandwich, etc. I do think "Tuna fish" has an appealing flow (euphonious consonants without any blends) and the ish pairs well with ich in sandwich

In my mind, tuna fish is the shredded stuff in a can and tuna is bigger pieces

Tuna fish is the chicken bird of the sea water.

Well, where I live, Tuna is also a cactus. Prickly pear is often called tuna. So yeah, tuna (fish) and tuna (fruit) can need disambiguation.

Some of you have never eaten tuna cow and it shows

Horse back riding

If you don’t specify, Americans will ride the wrong part of the horse

If we didn't say that, we wouldn't have the joke about the difference between a piano and a fish.

You can twist knobs on a guitar, but you can't make it drink.

I think it's mostly for the dad joke:

You can tune a piano, but you can't tune a fish.

What's the difference between a tuna and a piano?

We have to specify so that Jessica Simpson doesn't get confused with Chicken.

The opposite of "cow steak"

Tuna was not always popular and when people didn't know what it was it helped people know what they are buying. The US also having a large portion of bilingual people with a Spanish base, this helps it not get confused with cactus fruit (apparently tuna in Spanish)

Don't want to confuse it with a guitar tuna

"吞拿魚"

[transliteration of "tuna"] + ["fish"]

Tuna means prickly pear cactus fruit in Spanish, I've heard it called that pretty often in Texas

I’ve noticed the same thing with Koi. But not with trout or bass or most other fish.

It makes sense for swordfish, because just sword is ambiguous.

Language is weird.

Of note, the homophonic word, Tuna means the large fish with lots of mercury in it in English, and means the fruit of the prickly pear cactus in Spanish. I've made honey wine (mead or in this case melomel) with tuna, the fruit, and it's amusing to hear people's reaction to "tuna melomel".

its tuna fish because it refers specifically to the canned tuna fish sandwich and additional ingredients.

“Hand me that can of tuna please, I want to make a tuna fish sandwich. “

These words have come out of my mouth.

Yes they’re wrong. But something about the cadence.

Come to think of it, we do say "tuna fish sandwich", but we also say "tuna salad sandwich".

This entire thread is /c/badlinguistics.

Tuna is just the dish, tuna on a plate.

Tuna Fish is actually Tuna Salad. You would order a tuna fish sandwich (tuna salad), but you would not go to a restaurant and say "I will have the tuna fish" because that is just tuna.

Listen, y’all eat something called spotted dick - you can let us have tuna fish with no damn grief thank you

For the same reason that American cannibals eat human flesh

I just call it canned tuna. Or tuna from a can.

An educational article from the UK.

https://inv.nadeko.net/watch?__goaway_challenge=js-refresh&__goaway_id=b9170d881bbf10dbda83067dd8e00cd2&__goaway_referer=https%3A%2F%2Finv.nadeko.net%2F&v=5wSw3IWRJa0

The American English language is wierd... Two to four instances of one word to mean different things or to put emphasis on plurals or not... Plus certain connotations depending on who you're talking to. So trying to go and learn different languages, especially when they only have one instance of something, I have found.

They don’t have to say “tuna fish”. Most don’t, as far as I know. Who are you hanging out with that you think it’s somehow mandatory?

Why do they call their babies "Baby Joe" or "Baby Velma"? We know they are babies. Do they also call them "Young Adult Joe", and "Middle Aged Velma"?

I am having tuna fish as opposed to tuna steak.