A special group of microorganisms, which are called ammonia oxidizing bacteria (AOB), is capable to reduce ammonia-nitrogen from the ecosystem. However, this bacterium is slow growing and has a low yield capacity. Hence, the objective of this research was to grow the AOB using a support medium in the form of biofilters to reduce the ammonia-nitrogen. The enrichment cultures of this AOB were performed from the inoculum of common soil and effluent from fishpond. The experiment was conducted using two sets of flasks labelled Medium A and Medium B, respectively. Each medium contained three flasks of which Flasks 1 and 2 were inoculated with the common soil and effluent from a fishpond. Flask 3 was the control, which contained neither soil nor effluent from any fishpond or bio-filter. Then the ammonia-nitrogen and the formation of nitrite-nitrogen were measured. The results showed that the enrichment culture of the effluent of the fishpond had achieved a high formation of nitrite-nitrogen (450 µg/L) in comparison with the enrichment culture of the soil (360 µg/L). A high reduction of ammonia-nitrogen (50%) was obtained from the fishpond culture rather than the soil (40%). Secondly, a higher reduction of ammonia-nitrogen and production of nitrite-nitrogen were found in the bio-filter flasks against the controls (without bio-filter). In summary, it can be concluded that the bio-filter can be a reliable support to grow AOB
