Microbial community succession of home aquarium biofilters associated with early establishment of comammox Nitrospira

Nitrification in aquarium biofilters transforms toxic ammonia (NH₃/NH₄+) into less toxic nitrate (NO₃-) via nitrite (NO₂-). Known freshwater aquarium nitrifiers include ammonia- and nitrite-oxidizing bacteria, ammonia-oxidizing archaea (AOA), and complete ammonia-oxidizing Nitrospira (CMX), with CMX recently shown to dominate most freshwater aquarium biofilters. However, little is known about nitrifier succession during aquarium establishment in home settings. Based on CMX prevalence in mature aquariums and the rapid growth of ammonia-oxidizing bacteria (AOB), we hypothesized that AOB initially dominate before CMX establish. To test this, we monitored microbial succession and water chemistry in three home aquariums over 12 weeks, collecting weekly samples from aquarium water, biofilter beads, and sponge filters. Biofilter DNA was analyzed via 16S rRNA gene sequencing and quantitative PCR (qPCR) targeting amoA genes. Nitrification reduced ammonia and nitrite to undetectable levels by week 3 in two aquariums and by week 8 in the third. Ammonia oxidizer detection by qPCR coincided with the onset of ammonia oxidation, with AOA preferentially colonizing biofilter beads. Metagenomic profiling of week 12 biofilter samples confirmed AOA and comammox Nitrospira amoA genes in all aquariums, along with nxrB genes from both comammox and canonical Nitrospira nitrite oxidizers. These results provide insight into the establishment of ammonia oxidizers in residential aquariums. Future work should explore factors influencing nitrifier community assembly, including inoculation sources (e.g., live plants, biological supplements), fish load, and water chemistry.