Keywords: 
• biofilter media /
• recirculation aquaculture system /
• biofilm cultivation /
• NH₄⁺-N /
• ammonia nitrogen /
• water treatment

Abstract: Five biofilter media,bacterial ball,white glass ring,volcanic rock,red breathing loop and coral sandstone,were used to remove dissolved NH₄⁺-N,NO₂-N and NO₃^--N in the seawater in a closed recirculation aquaculture system by using the natural biofilm cultivation method,and to examine the change of and the relation among NH₄⁺-N,NO₂-N and NO₃^--N in the seawater after the biofilm maturation. During biofilm cultivation,these biofilter media were found to reduce the NH₄⁺-N concentrations over time,increase initially and then decrease the NO₂-N concentrations,and increase gradually the NO₃^--N concentrations. The coral sandstone and the red breathing loop were better than other media in removing inorganic nitrogen. They formed biofilms in about 23 days and reduced the ammonia concentrations to 0. 215 mg·L^-1 at day 15 after biofilm formation,with a NH₄⁺-N removal rate of upto 98. 5%. After biofilm maturation,the exponential equation y = ae^-bx was fitted to the data of the NH₄⁺-N removal efficiencies of each biofilter medium,and the equation y = 1/（a+b/xlnx +c/x） fitted to the data of the NO₂-N and NO₃^--N removal efficiencies. Moreover,the relationship between the NH₄⁺-N concentration and the NO₂-N concentration or the NO₃^--N concentration in the sea water was also fitted by using the exponential equation y = ae^-bx.