A closed ecosystem produces tomatoes and fish in northern Sweden
By growing fish and tomatoes in the same ecosystem, the fish produce fertilizer for the tomatoes and the tomato plants clean the water for the fish. With this system, 1 kilo of fish food produces 1 kilo of fish and 10 to 12 kilo if tomatoes. The video is in Swedish but you can see some of the facilities.
A similar type of technology has also been implemented in some places in the UK if I remember correctly. They use the same techniques on rooftops so that you can farm in a sustainable manner in urban centres.