A Berlin -based company, Efficient City Farming (ECF), has developed the Container Farm, which they hope will be a step in the right direction towards a more sustainable concept of urban living. Tomato plants are capable of growing in the containers. They are surrounded by water in which tiny perch swim and absorb the nutrients excreted by the fish as waste. It is called an Aquaponic system, which is a close circulatory system where the fish and plants can grow in a symbiosis. That means that the fish produce the fertilizer for the plants and the plants clean the water for the fish.
Essentially all an urban farmer would need to add is fish food for the production; the rest is taken care of by Mother Nature. Aquaponic systems are not in themselves new; various varieties of them have been in development for almost two decades. But it is only recently that the system has become efficient enough and easy enough to operate, but is not really economically viable yet.