A greenhouse is not only conducive to the growth of tomatoes, but is also a place where fish such as the Nile perch can thrive, because both like heat. Researchers have taken advantage of this fact and developed a sustainable method that can be used for urban farming. In their patented facility, the nutrient-rich wastewater from the fish tank is purified and subsequently used to water vegetables. With this system, water is conserved during production and there are virtually no emissions. In the future, tomatoes and fish can be cultivated in the middle of cities and served fresh directly from the source.