Look at this very short animation describing the Aquaponic concept.
Aquaponics is a food production system that integrates standard aquaculture (increasing aquatic creatures such as for instance snails, fish, crayfish or prawns in tanks) with hydroponics (cultivating flowers in liquid) within a symbiotic environment. In normal aquaculture, excretions from the animals being raised can accumulate in the water, increasing toxicity. In an system that is aquaponic water from an aquaculture system is given up to a hydroponic system in which the by-products tend to be separated by nitrification bacteria into nitrates and nitrites, that are used by the plants as nutritional elements. Water will be recirculated back into the aquaculture system.