Is eating canned fish healthy or unhealthy ?

I would say it is: unhealthy due to the amount of salt/brine that is used as well as any spice/preservatives for packaging purposes. You would have to convince me very well to eat canned tuna instead of fresh tuna, though I understand they are used for different purposes in cooking. Any extra salt is detrimental to our blood vessels and promotes hypertension, stroke, and heart attacks.