The study reports the discovery and characterization of two *Nitrospira* species that can completely oxidize ammonia to nitrate in a single organism, a process known as complete nitrification or comammox. This process was previously thought to be catalyzed by two distinct functional groups: ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB). The two *Nitrospira* species, named *Candidatus* Nitrospira nitrosa and *Candidatus* Nitrospira nitrificans, were enriched from an ammonium-oxidizing biofilm in a recirculation aquaculture system. They were found to possess all the necessary enzymes for ammonia oxidation, including a unique form of ammonia monooxygenase (AMO) enzyme. These organisms were capable of complete nitrification under hypoxic conditions, as evidenced by the production of 15N-labeled nitrogen gas. The presence of these organisms was confirmed through in situ labeling experiments using fluorescein thiocarbamoylpropargylamine (FTCP), which specifically binds to the AMO enzyme. The study also highlights the environmental distribution of these novel *Nitrospira* species, suggesting their presence in various habitats, including engineered and natural environments. The findings challenge the traditional view of nitrification as a two-step process and provide new insights into the biogeochemical nitrogen cycle.The study reports the discovery and characterization of two *Nitrospira* species that can completely oxidize ammonia to nitrate in a single organism, a process known as complete nitrification or comammox. This process was previously thought to be catalyzed by two distinct functional groups: ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB). The two *Nitrospira* species, named *Candidatus* Nitrospira nitrosa and *Candidatus* Nitrospira nitrificans, were enriched from an ammonium-oxidizing biofilm in a recirculation aquaculture system. They were found to possess all the necessary enzymes for ammonia oxidation, including a unique form of ammonia monooxygenase (AMO) enzyme. These organisms were capable of complete nitrification under hypoxic conditions, as evidenced by the production of 15N-labeled nitrogen gas. The presence of these organisms was confirmed through in situ labeling experiments using fluorescein thiocarbamoylpropargylamine (FTCP), which specifically binds to the AMO enzyme. The study also highlights the environmental distribution of these novel *Nitrospira* species, suggesting their presence in various habitats, including engineered and natural environments. The findings challenge the traditional view of nitrification as a two-step process and provide new insights into the biogeochemical nitrogen cycle.