The aim of this study was to analyze the process and maturation of moving bed biofilm reactors MBBR without bacterial inoculation, evaluating the biofilms and investigating their viable bacterial colonies for 122 days. Two bioreactors coupled in series received water from fish culture tanks that housed the species Oreochromis niloticus. . The successful maturation of the bioreactors occurred around 100 days into the experiment, when the nitration rate increased. We identified 105 species in the biofilms, which were grouped into 65 genera, 3 of which were beyond addition: Pseudomonas, Nitrospira and Gemmobacter. Nitrospira was identified as the main bacterial genus responsible for the nitrification process.