Hi If your can is 20years old, I would not eat it.
I have a All american canning machine and use it with the pressure cooker to
sterlize and cook the salmond I chatch.
But would not eat it if it was 3 years old.
Like all cans make sure they have the vaccum on the top and not blown. Canning fish is quite eazy and lasts for up to 3 years.
Hope this helps you but can make your self sick or die if it is off