ABSTRACT The mortalities for aquaculture animals in a newly established pond tend to be high. The phenomenon can be described as the ¡§new pond syndrome¡¨. The main reason for the phenomenon is still unclear. However, it is suspected that the accumulation of ammonia due to insufficient nitrifying bacteria in the system may have been the cause. The purposes of this thesis are to test the theory and to rectify the problem by introducing a nitrifying trickling filtration system using E. amblycephalus as the experimental animal. It was found that the ammonia concentrations did increase to the levels detrimental to the newly hatched fries within 6 days. The LD50 of ammonia to the young fries was determined to be 0.034 ppm in 48 hr. The nitrifying filtration system could reduce the ammonia concentrations substantially and raise the breeding rates for the fries. On two separate trials, one with fish tanks in the laboratory and the other in the ponds of a fish farm, 12.3% and 4.2% of the newly hatched fries were still alive after 34 days, respectively, as compared to 0% and 0.035% for the controls. Meanwhile, a pH value of equal to or greater than 8.6 was detrimental to the fries. Therefore, the low breeding rates for the E. amblycephalus fries may have been caused by the ¡§new pond syndrome¡¨ due to the toxic effects of accumulated ammonia in the early stage of the pond usage. Removal of ammonia with the filtration system could increase the survival rates for the fries.