The fish feed of today contains far less animal protein than it used to - a lot of the protein now comes from soy.
Salmon farming, for example, can be incredibly efficient - Norwegian salmon farms typically get 1 kg of fish for every 1.15-1.2kg of feed. That's an amazing conversion factor.
That said, there are serious welfare concerns with fish farming. Even in Norway, which has some of the highest standards in the world, sea lice are a huge problem, and the mechanical and chemical treatments used are horrible. Another issue is the welfare of "cleaner fish", which are places in cages to eat lice from salmon - AFAIK there are still no regulations around their welfare, and mortality rates are shocking[0]
Maybe this information is out of date, but these are the issues I was referencing:
Anchovies, sardines and other palm-sized, schooling fish are caught in the ocean and processed into fishmeal and oil to feed to other fishes and crustaceans that are raised in aquatic farms around the world.
Every year, millions of tons of wild fish, like sardines and anchovies, are caught and processed into fishmeal and fish oil, which is used to make feed for farm-raised species.
It's true that a lot of wild catch still goes towards fishmeal; my point was simply that things are improving a lot there, at least for salmon (I presume it's the case for other species too, but I don't know that).
We go through this whole process and the result is half as much fish as we started with, and it's worse for you.