Tilapia are protein producing machines. They convert 1.1-1.3 lbs of feed to 1 lbs of protein in 9 months (some factors like temperature, feed rate at different stages of life, and other things affect this ratio). But here's the real kicker--by providing the nitrogen that crops need, the crops can then turn CO2 into into calories via photosynthesis. Super high quality produce becomes the extremely valuable byproduct, effectively, of aquacultured seafood.
From a global net energy perspective, this is huge. Where do you think that nitrate fertilizer for soybeans is coming from? The Haber-Bosch process is crazy energy expensive. Growing soybeans for protein also has huge environmental costs--see deforestation in Brazil for details. 80% of cultivable land on earth is already in use, but food production needs to increase by 70% by 2050. Protein production will need to double due to the rising global middle class. And that middle class wants fish.
On top of all that, there's the fact that people don't want to eat just soybeans for protein. There's now more seafood consumed globally than beef cattle. By 2030, 62% of all seafood will be aquacultured, and there are some suggestions that most of the wild seafood species we know will be either extinct or heavily depleted.