You can always go for canned salmon, or fresh, it's pretty cheap. Plus you get the omega-3's. I give my dogs tuna too, but always make extra for them when salmon is served.
I heard a radio show about the mercury... they said that "farm raised" had MORE mercury than fresh caught... it was a pregnant woman asking how much fish and what kind of fish she can eat. They said 2 to 3 servings a week should be fine. I don't remember all the fish that has the higher doses of mercury.... swordfish... I know of.... Anyone who knows more, please post.