NOVELTY - A water recycling zero-discharge type aquaculture system comprises an aquaculture tank (10), a water-treatment section (100) for treating water, a foam fractionation unit (20) disposed downstream of the aquaculture tank, an anaerobic pond (30) downstream of the aquaculture tank and comprising an oxygen concentration reduction portion (36) adapted to reduce the concentration of dissolved oxygen in the water and pollution anaerobic treatment portion (38) downstream of the oxygen concentration reduction portion for anaerobically treating the water.    USE - Water recycling zero-discharge type aquaculture system is used for treating aquaculture water (claimed). Uses include but are not limited to municipal wastewater treatment systems, grey water treatment systems, livestock wastewater treatment systems, industrial wastewater treatment systems, hydroponics water recycling systems, and systems for anaerobic manufacturing of biogas.    ADVANTAGE - The system has high water-treating efficiency, and removes toxic pollutants from water which causes harm to fish.    DETAILED DESCRIPTION - A water recycling zero-discharge type aquaculture system comprises an aquaculture tank, a water treatment section for treating water, a foam fractionation unit disposed downstream of the aquaculture tank, an anaerobic pond downstream of the aquaculture tank and comprising an oxygen concentration reduction portion adapted to reduce the concentration of dissolved oxygen in the water and pollution anaerobic treatment portion downstream of the oxygen concentration reduction portion for anaerobically treating the water, an algae pool (40) downstream of the foam fractionation unit and the anaerobic pond, a common pool (50) downstream of the algae pool, bio-filtration reactor (60) downstream of the common pool and oxygen injection unit (70) downstream of the bio-filtration reactor and upstream of the aquaculture tank. INDEPENDENT CLAIMS are included for the following:    (1) process of treating water in aquaculture system;    (2) anaerobic pond for anaerobically treating water;    (3) method of treating water having pollutants and dissolved oxygen under anaerobic conditions; and    (4) method of maintaining concentration of dissolved oxygen in water to less than threshold level throughout all layers of the water.    DESCRIPTION OF DRAWING(S) - The drawing shows the schematic structure of system.    Aquaculture tank (10)    Foam fractionation unit (20)    Anaerobic pond (30)    Oxygen concentration reduction portion (36)    Pollution anaerobic treatment portion (38)    Algae pool (40)    Common pool (50)    Bio-filtration reactor (60)    Oxygen-injection unit (70)    Water-treatment section (100)