Molecular research, using culture-independent methods, on samples taken from many RAS has demonstrated that there are multiple microorganisms and pathways involved in the oxidation of ammonia to nitrite and nitrite to nitrate, commonly called nitrification. There may be no ‘one-size’ fits all solution to rapidly starting-up a bio-filter. The microorganism community responsible for nitrification in RAS varies depending upon ambient water quality factors as least in the first several months after start-up. However, obtaining a stable biofilter, defined as one with minimum fluctuations in ammonia and nitrite concentrations may depend more on providing a stable biofilm environment rather than a hard set of water chemistry values. Research has also shown that as a bio-filter ages there may be a succession of microorganisms responsible to the nitrification process. Knowledge of these factors will assist RAS operator start-up, maintain and troubleshoot biofilters in their RAS.