Abstract: Abstract:A lab-scale cyclobio fluidized sand biofilter was designed to study biofiltration in recirculating aquaculture systems. First, the biofiltration start was studied separately in fresh water and seawater recirculating aquaculture systems. Second, the influence of different operation conditions on water treatment were studied by natural membrane-forming, including three bed expansions (50%, 75% and 100%) and four carbon nitrogen ratios (0, 0.5, 1 and 2). A synthetic substrate solution was continuously fed into the tanks after measuring of water quality every time. The results showed that the total ammonia-nitrogen concentration was decreased, fluctuating lightly in later period. The nitrite-nitrogen concentration was in a shape of clock, increasing quickly first and keeping the maximum for days then decreasing quickly, fluctuating lightly in later period. Not only ammonia-nitrogen and nitrite-nitrogen concentrations, but also the stability of water quality should be as the criteria of completion judgement of biofiltration start. In these conditions, biofiltration start of cyclobio fluidized biofilter were 47 days in fresh water system and 60 days in seawater system. Besides, we found that the cyclobio fluidized biofilter had little help in removing phosphorous, maybe because dissolved oxygen was always more than 6 mg/L in system. The nitrate-nitrogen concentration was increased on the whole. However, high nitrite-nitrogen concentration may harm fish health. To avoid that, there were two control methods: exchanging some new water to system every day, or adding denitrification equipment to recirculating aquaculture systems. Meanwhile, the results of operation conditions showed that the maximum load of system was separately 2.5, 3.0 and 3.5 mg/L with different bed expansions 50%, 75% and 100%. And the removal efficiency of total ammonia-nitrogen increased and the nitrite-nitrogen concentration decreased with the enhancement of bed expansion. The efficiency of ammonia-nitrogen was the highest and the load of system was the largest under 100% bed expansion in this study, while the volumetric total ammonia nitrogen conversion rate was 881g/(m3∙d). Besides, inhibition of organic on nitrification was observed. The ammonia-nitrogen, nitrite-nitrogen and COD concentrations were increased with the enhancement of ratios of carbon and nitrogen. Especially, water quality went bad obviously when ratios of carbon and nitrogen was more than 2.0. The ammonia-nitrogen and nitrite-nitrogen even reached to lethal or medial lethal concentrations of some fishes. The possibility was that heterotrophic microorganisms breed quickly with the enhancement of ratios of carbon and nitrogen and autotrophic nitrifying bacteria was embedding and died absenting of oxygen. This research can provide a reference for starting, operation and management of cyclobio fluidized biofilters in production