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Biological filtration used in the recirculating aquaculture systems comprises mostly of nitrification process.The nitrification biofilter removes ammonia and nitrite by converts them to nitrate.Since nitrate has lower toxicity,water can be reused or recirculated in the aquaculture system as long as nitrate concentration is below the limitation concentration(e.g.50 mg-N/L).However,to set up the ready-to-used nitrification biofiher usually requires long incubation period(e.g.20-40 days)and the biofilter must be acclimated in proper condition to maintain nitrification efficiency.This study examined the effect of ammonia concentration on ammonia removal rate of the acclimated nitrification biofilter.The experiment units were consisted of three bioreactors,each containing 5 L of 30 psu synthetic seawater and 100 pieces of sterilized plastic biofilter media(BCN-009,2H,Germany).Before starting the experiment,starter biofilter media was pre-incubated for 1 month to achieve nitrifying bacteria population.Active biofilter media were later placed in the reactors to introduced nitrifying bacteria to new biofilter media.During the experiment,different concentrations of ammonium chloride i.e.0,0.5 and 2 mg NH4-N/L was continuously fed into the reactor at 144 mL/day using peristaltic pump.Calcium carbonate(CaCO3)at 100-120mg/L was added as inorganic carbon for nitrifying bacteria.The operating temperature was maintained at 25 ℃.Ammonia removal and nitrification rates of the biofilter media were regularly monitored by two approaches.The first approach was to monitor ammonia,nitrite and nitrate concentrations in water inlet and outlet of the reactor.The second approach was performed by taken out the biofilter media from the reactor and measured nitrification rate in batch test.The result showed that ammonia concentration during nitrification biofilter acclimation period strongly affected nitrification rate of the biofilter.Fluidized biofilter from the continuous reactor fed with 0,0.5,2 mg-N/L ammonia for one month had ammonia removal rates of 36.8,35.4 and 50.1 mg-N/m2/d,respectively.After two months acclimation,biofilter had ammonia removal rates of 35.3,40.7 and 47.2 mg-N/m2/d,respectively.Scanning electron micrographs of bacteria attached to the biofilter surface showed that higher density of bacteria was found in higher ammonia concentration.It could be summarized that high ammonia concentration e.g.2 mg-N/L significantly stimulated higher nitrification rate and reduced the time require for biofilter acclimation,however,statistical analysis(t-test)suggested that acclimation period did not significantly increase ammonia removal rate of the biofilter.