only way to manage the fish/mercury issue is to eat the bigger fish (halibut, tuna, swordfish) only once a week, and if eating more frequently, eat smaller fish and farm-raised fish.
Fish are essential because of the fatty acids. Can't say enough about how important Omega 3 and Omega 5 fatty acids are; I get them via flax seed oil, but fish oils make the EFA's more bio-available.
I've heard of a supplemnt that is called "Pharmeceutical grade fish oil" that has all metals and toxins removed. Generally, fish oil supplements are highly contaminated and I won't take them, but if I find this new product I'll get it for sure.