High density of tilapia production by biofloc farming system aims to get more produce and save food costs because fish can eat floc as food. Maintaining sludge and maintaining nutrient levels is essential. Therefore, this research aimed to study the effect of carbon: nitrogen (C:N) ratio in ponds on the accumulation of off-flavor. The experiment was designed for 3 trials. Experiment 1 studied the effects of C:N ratio on accumulation of off-flavor, phytoplankton and water quality in 2 private farms: Baan Mae Kung Luang, Thung Tom Subdistrict, San Pa Tong District, Chiang Mai Province (Farm 1) and Ban Huay Som Farm, San Klang Subdistrict, San Pa Tong District, Chiang Mai Province (Farm 2). Experiment 2 compared the effects of tilapia culture at a carbon to nitrogen (C:N) ratio of 14: 1 (T1) and a carbon: nitrogen (C: N) at 10: 1 (T2) on growth performance and the accumulation of muddy smell in the water. This trial was conducted in a 2-ton pond cultured over a period of 90 days. Experiment 3 compared methods for reducing the off flavor in conventional biofloc tilapia culture and conventional system followed by changing to 1-month clear water before harvest. The results of the first experiment showed that the water from the fish ponds in Farm 1 and Farm 2 accumulated mud odor with mean gosmin values of 0.10 ? 0.18 ?g / l and 0.31 ? 0.11 ?g / l, respectively and the mean MIB value of 0.12 ? 0.21 ?g / l and 0.18 ? 0.19 ?g / l, respectively. The settled floc in farm 1 and farm 2 were 4.58 ? 4.13 ml / l and 1.20 ? 0.62 ml / l. The phytoplankton that produced the mud smell was Anabaenopsis sp. and were found heavily in both farms. Experiment 2 showed that the muddy smell in tilapia ponds water was as high as 0.10 ?g / l. Settled floc in T1 and T2 were 52.64 ? 1.84 and 34.73 ? 13.9 ml / l, respectively. Growth rate and feed conversion ratio were significant difference (P <0.05). it was found that production cost of culture fish by biofloc with a carbon: nitrogen ratio of 14: 1 was lower than that of carbon-to-nitrogen fish. 10:1. Moreover, raising fish with traditional biofloc in combination with clear water system before selling can reduce the muddy smell in the water.