Tilapia are low on the food chain and adaptable—basically, easy to cultivate. And people have been doing so for years: a bas-relief on a 4,000-year-old Egyptian tomb shows tilapia held in ponds. Today nearly all tilapia farmed in the U.S. are raised in self-contained aquariums that purify and recycle water. These so-called recirculating aquaculture systems often employ “biofilters”—microorganisms that feed on nitrogen—to treat wastewater. Bacteria break down some fish waste into nitrogen (which the microorganisms absorb for fuel) and other organic compounds that can be used to grow plants and algae, which are fed back to the fish. Sediment is removed from the tanks mechanically, and 99 percent of the water is recycled. “It’s a highly efficient system,” says J. Emmett Duffy, Ph.D., professor of marine science at The College of William and Mary in Gloucester Point, Virginia.
Or in laymen's terms, their poop fertilizes their crop of algae.
