Farmed fish is mostly fed with industrially caught wild fish (the species and sizes that don't fetch a good market price for direct consumption), they are not a solution to overfishing or the feeding of billions at all. Traditional farming of freshwater fish fish is an exception, but I don't see carp reaching global mass appeal any time soon (and it quite likely either would not scale or it would turn nasty as well from scaling, just like everything we touch in search for excessive yields)