I can't believe this, but I actually have to defend Wayner, and we all know that he was on my "Ignore List" for quite some time, but he actually has been listening to advice lately.
That being said, I wouldn't dream of feeding canned tuna. The stuff in the pouch, maybe .. but never canned. The lining of most cans contains lots of chemicals and horomones that are causing much controversial debates as to whether or not they are harmful to a humans health. My thought here is that if there is a risk to it being harmful to something that weighs over 100lbs, it would have to be even more risker to give to something that is only a few pounds (if that) such as a fish.
Even just tuna and fish, no preservatives (which I don't think canned is going to offer, FYI), in a can is going to have this protective lining.