In addition to fish farming, woven baskets filled with shellfish can be hung below the fish which will feed on the nutrients in the water, both algae and fish droppings. Everything from clams to mussels to scallops can be grown in these baskets. Even oysters, which produce pearls! All of this can be done sustainably using renewable energy sources like OTEC and solar, and keeping a self contained carbon cycle to increase growth.
Lastly, because of biorock (which we also covered in a previous blog post here) we can even grow calcium based deposits on the bottom of seasteads, which can have young corals fixed to them. These corals will grow into the structure itself and become full sized coral reefs, providing shelter and food for all kinds of wild animals. With this in mind, there is truly no limit to what kinds of environments can be grown on seasteads. Each floating city could have a different type of environment, some based on coral, others on seagrass, and many will probably be mixed, but all will produce a plethora of resources for their inhabitants.
The seasteading movement has a lot of potential benefits to humanity, from increasing living space, and decreasing real estate costs, to increasing fish production while building up our environment. According to the UN, Earth will have between 9.1 and 10 billion people by 2050. Those people will still need to eat and have a place to rest, and seasteading can provide the solutions to their problems, all while growing new coral reefs and other deep sea environments in the process.