I have fish about twice a week, which I think is a healthy, balanced amount.
I wouldn't have it every day.

Don't get the preoccupation with tinned tuna at all. It's ok once in a while, and with something else; some people at work eat a whole tin every day, practically on its own.
In fact I would personally avoid too much tuna and too much shellfish

I don't know about fish, but Timothy West ate a lot of royal jelly and that ultimately had a detrimental effect on him. I suppose too much fish could be a similar thing.