Nitrification is a critical process for preventing ammonia toxicity in engineered biofilter environments. This work describes the cultivation and complete genome sequence of a novel AOA representative enriched from a freshwater aquarium biofilter. In addition, despite the common belief in the aquarium industry that AOB mediate ammonia oxidation, the present study suggests an
in situ
role for “
Ca
. Nitrosotenuis aquarius”-like AOA in freshwater aquarium biofilters.