Fig. 4. Nitrogen cycle in extensive aquaculture ponds with substrates for periphyton growth. The nitrogen cycle is similar to that of in water ponds with a long hydraulic residence time, but now TAN concentration does not build up in the water column, neither is the nitrate concentration. The periphyton community takes up both TAN and nitrate and edible biomass is formed. The cultured fish can graze on the periphyton community and hence nitrogen, originating from wasted feed and excretion by fish, is redirected towards the cultured organism and therefore, this technique enhances the overall efficiency of nutrient conversion of feed.