Fish is really good for you! An excellent source of omega-3, protein, and it is a pretty lean protein in terms of calorie/ounce.
Two rules of thumb for you when it comes to mercury in fish:
The higher up the food chain (and the bigger the fish), the more mercury.
Two 6 ounce servings of fish a week (or 12 ounces total)is a good amount to aim for. That is a good threshold amount to get the benefits of eating fish but also to allow your body to naturally eliminate mercury.
Also, unless your diet is primarily fish (eating it three meals a day), mercury in fish is really only a concern for children, women who are pregnant, or women who plan to get pregnant shortly (child bearing age).
Some interesting reading:http://www.csa.com/discoveryguides/mercury/review.php
This as been discussed in other forum posts as well. Check-out the below links to see what has already been discussed.Diet Talk: Mercury in FishHow much fish can you eat in a dayDiet Talk: Fish for Fish Haters