“You can tune a piano, but you can’t tuna fish” was the title of REO Speedwagon’s double platinum-selling 1978 album. The title is from an old riddle: “Question: What’s the difference between a piano and a fish? Answer: You can’t tuna fish.”
   
The joke is cited in print from at least 1964. “How do you tune a fish?”/“With its scales” is a related riddle.
   
 
Wikipedia: You Can Tune a Piano but You Can’t Tuna Fish
You Can Tune a Piano but You Can’t Tuna Fish, REO Speedwagon’s seventh studio album, was the first to be produced entirely by lead singer Kevin Cronin and lead guitarist Gary Richrath. The album was REO’s first to make the Top 40, peaking at #29. The album sold over 2 million copies in the U.S. which led it to go 2x Platinum.
 
21 March 1964, Pittsburgh (PA) Courier, “The ‘Midnight Man’ In Chicago: Streamlined Press Express” by Ted Watson, pg. 13, col. 6: 
... and as Bob Bryant, the Brown Derby mixologist, would say: “The difference between a piano and a fish is that you can’t tuna fish.”
     
18 August 1965, Evening Journal (Wilmington, DE), “Man About Town” by Carl G, Smith, pg. 25, col. 1:
Today’s Horror
What’s the difference between a piano and a fish?
You can’t tune a fish.
 
24 June 1968, San Diego (CA) Union, “Fun Time—The Riddle Box,” pg. C-8, col. 2:
What’s the difference between a piano and a tuna fish?
You can’t tune a fish.
 
3 September 1969, Alton (IL) Evening Telegraph, pg. A12, col. 2:
What’s the difference between a fish and a piano?
You can’t tune a fish.
   
15 August 1973, Capital Times, (Madison, WI), Green section, pg. 1, col. 5:   
What’s the difference between a fish and a piano?
You can’t tuna fish.
   
Google Books
Biggest Riddle Book in the World
By Joseph Rosenbloom
Illustrated by Joyce Behr
Edition: illustrated, revised
Published by Sterling Publishing Company, Inc.
1976
Pg. 39:
What is the difference between a fish and a piano?
You can’t tuna fish.
   
24 February 1977, Provo (UT) Daily Herald, pg. 24, col. 2:
What’s the difference between a piano and a fish?
You can’t tuna fish.
   
Google News Archive
8 November 1977, Free Lance-Star (Fredericksburg, VA), pg, 14, cols. 1-2:
What’s the difference between a piano and a fish?
You can tune a piano but you can’t tuna fish.