I know that fish, especially oily fish, is very good for you, but I got quite concerned about mercury in my last pregnancy and did quite a lot of research on it. Luckily the 2 types of fish I really like, sardines and tinned salmon (tinned because it's always wild, not farmed), are about the safest you can eat.
Anyway today I came across this online mercury calculator and thought I'd link to it here in case anyone else finds it useful!