The measurement on the conversion rate of nitrifying bacteria to NH 4 N were determined in the filtering trouphs of Takingfugu obscurus. The nitrifying rate of nitrifying bacteria was understood quantitatively. The results showed that the quantity of the nitrifying bacteria in the sands of the filtering trouph was 21 times greater than that in the normal water, 10 times greater than that in the controlled group. The nitrifying rate of the sands was 100 times greater than the normal water. The major factors affecting the nitrifying bacteria to NH 4 N are temperature, DO and biomass bacteria. Therefore, it is indicated that filtering trouphs can effectively self purify the fish pond environment.