Development of a DHS-USB recirculating system to remove nitrogen from a marine fish aquarium

•A recirculating nitrogen removal system was developed to treat marine aquarium water.•The optimum C/N ratio for denitrification was 1.2.•Autotrophic and heterotrophic denitrifiers were detected in the reactor.•There were nitrification bacteria in the sponge of the reactor. A recirculating nitrogen (N) removal system comprising a down-flow hanging sponge (DHS) reactor combined with an up-flow sludge blanket (USB) reactor was developed. We used this system in a fish-breeding marine 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 below 30mgN/L, and total ammonia nitrogen and nitrite concentrations were below 0.1mgN/L. The optimum denitrification ratio was approximately 1.2. Analysis of the microbial community showed that there were ammonia-oxidizing bacteria and nitrite-oxidizing bacteria in the DHS reactor, and that there were two types of denitrifying bacteria in the USB reactor, namely heterotrophs and autotrophs, that removed nitrate from the marine aquarium water.