We all know that fish is one of the superfoods, packed with protein for muscle building and those all-important omega-3's that help with mood, inflammation, heart disease, the brain and potentially free testosterone. However, it is rare to find an expert who advocates eating fish on a consistent basis. In fact, the great majority would caution against fish consumption more than once or twice per week. (Read my link on Fish and Fish Oil for more details.)
NEWS FLASH: One recent study shows that taking fish oil capsules dramatically increases the risk for advanced prostate cancer - this is the kind you don't want! - and significantly increases it for all prostate cancers. See this article for more details. Based on this, the authors recommended eating just one or two meals of fish per week instead. Keep in mind that this is just one study, but the results were so pronounced that it should be carefully considered. For a nice rebuttal to this study, read this discussion for more information.
The reluctance to recommend even moderate food consumption is with good reason: methylmercury. For example, one famous actress, Daphne Zuniga, ate sushi four time per week and found herself with mercury poisoning and the ensuing "weak memory, headaches, crying spells, skin rashes and low grade depression".  Furthermore, the New York Times recently ran a piece that explained how badly contaminated Manhattin sushi was. Their findings were that "at most of [the restaurants], a regular diet of six pieces a week would exceed the levels considered acceptable by the Environmental Protection Agency. Sushi from 5 of the 20 places had mercury levels so high that the Food and Drug Administration could take legal action to remove the fish from the market". 
CAUTION: Kids can get dangerous levels of mercury from eating the wrong kind of mercury. Canned (albacore) tuna is a large fish that accumulates about three times the level of mercury as the canned light variety.  The brains of children are particularly vulnerable to mercury and experts believe that even relatively small amounts of mercury can
Rest assured, it's not just sushi that has high levels of mercury: many varieties of fish have been affected. Freshwater fish in the U.S., for example, have been found to be loaded with mercury and PCB's and dioxin as well. Ocean going fish are often no better, especially shark, swordfish and king mackeral. 
The problem is unlikely to go away anytime soon as the methylmercury comes from the emissions of coal fired plants throughout the globe. Fortunately, most of us can detoxify mercury in about a month or a little more, but what is frightening is that a significant block of us carry a gene that retains mercury for about six months, making its impact much more profound and long lasting.
One Discover reporter had his mercury levels measure at 4 ug/l, a little below the level, 5.8 ug/l, considered the safe threshhold by most experts. However, when he ate fish, his blood mercury levels spiked to 12 and 13 ug/l, a very dangerous level.  As expected, he found that he had the negative gene that affected glutathione activity, glutathione being the body's natural antioxidant that detoxes mercury and many other dangerous chemicals and
By the way, some would argue that the risk of fish consumption is grossly overstated. After all, a huge percentage of people in Japan eat a boatload of fish and have very high tissue mercury levels and yet have shown no ill effects.  Again, though, I would argue better safe than sorry as there is no compelling reason to consume fish more than once per week, considering that there are excellent brands of fish oil out there.
Another option is to eat the five varieties of fish recommended by the EPA as relatively low in mercury: canned light tuna, salmon, pollock and catfish.  However, caution should be exercised in the case of farm raised catfish as they have significant levels of inflammatory agents according to Chilton's book Inflammation Nation.