This study aims to analyze the value of LC50-96 hours of tannery waste exposed to Oreochromis niloticus and see changes in the level of oxygen consumption and the opercullar respiratory rate. LC50-96 hours using Whole Effluent Toxicity (WET) method - static test. For the measurement of oxygen consumption level and operculum movements were conducted at 1.85% and 3.69% of wastewater exposure. Observations were made for 30 days after the acclimatization period. Data analysis used regression and correlation analysis and ANOVA significance test. The results showed LC50-96 hours was 25.85%. The level of oxygen consumption of O. niloticus at concentration of 1.85% and concentration 3.69% were decreased in the duration of exposure and the control treatment was increased with P value <0.05. While the frequency of the opercullar respiratory rate at concentration of 1.85% and 3.69% were increased also in control experiment during 30 days of tannery wastewater exposure, with the P value <0.05. The longer of the exposure time and the higher the concentration of wastewater, the decreasing of oxygen consumption level and the increasing of opercullar respiratory rate.