There is a way to "clean" fish of heavy metals. Yes fish (especially the bigger ones - e.g. swordfish, tuna, shark etc) do contain mercury, but it is nothing compared to the mercury that is leeching out of your teeth this very second.
The way is the natural way which several native diets have discovered (e.g. the mediteraneans) which is to cook fish with garlic. Garlic contains thiols (sulphur compounds) which are strongly attracted to mercury and other heavy metals. They will quickly bind to the mercury rendering it harmless, and it can then be escorted out of the body.
I read about this tip in Dr Blaylock's "Nutrition secrets that may save your life" which I thoroughly recommend.
So every time you cook fish, just throw a couple of cloves of garlic in for good measure. I've started doing this regularly, and I must say just from a taste point of view it is great. Also I feel better for it.