A recirculating nitrogen (N) removal system comprising a down-flow hanging sponge (DHS) reactor com-bined with an up-flow sludge blanket (USB) reactor was developed. We used this system in a fish-breedingmarine aquarium to evaluate the N removal and to determine the optimum C/N ratio for denitrification.With this system in place, nitrate-nitrogen concentrations in the marine fish aquarium remained below30 mg N/L, and total ammonia nitrogen and nitrite concentrations were below 0.1 mg N/L. The optimumdenitrification ratio was approximately 1.2. Analysis of the microbial community showed that there wereammonia-oxidizing bacteria and nitrite-oxidizing bacteria in the DHS reactor, and that there were twotypes of denitrifying bacteria in the USB reactor, namely heterotrophs and autotrophs, that removednitrate from the marine aquarium water.