Probably not a well known place for fish...in Flores. The place is absolutely full all the time. They have fresh seafood that they sell at a really good price. They will cook it for you in the moment to go in whatever way you wish, sell it fresh, and they also sell already prepared seafood to go. They have all kinds of fish. You can find the place by taking the subway down to plaza flores (or a bus), and get off there and ask (I forget the exact location, just ask for the pescaderia and everyone will know). It's only about two or three blocks from there. Believe me, you will get better prices than the places listed above. Good white salmon there, too.
If not, there's also a good one in Caballito, in Pedro Goyena and Senillosa, around that area. They have decent variety as well. Can't top the one in Flores though.