In aquaculture systems, a lack of water exchange, continuous input of food, and bacterial growth in culture tanks, can cause an increase in floc concentrations. Increased floc concentrations increase oxygen demand and clog the gills of cultured animals. Therefore, floc concentration is one of the most important management factors in a floc aquaculture system. The African catfish () is a candidate for biofloc aquaculture systems due to the ability to adapt to the adverse water conditions. No previous studies have investigated the effects of floc concentrations on African catfish cultured in biofloc aquaculture systems. The current study investigated the effect of different floc concentrations on the water quality, bacteria community compositions, nitrogen budget, and growth performance of juvenile African catfish in biofloc systems for 140 d. Two treatments were referred as low floc concentration (LF) or high floc concentration (HF), with an average concentration of 561.18 mg/L and 780.41 mg/L, respectively. The results suggested that there were no significant differences in average concentrations of total ammonia nitrogen (TAN), nitrite nitrogen (NO₂^--N), growth performance of the farmed fish, and nitrogen budget items between the treatments (-N) in HF (822.0 mg/L) was significantly higher than that of LF (623.33 mg/L), which suggested that the nitrification process was ongoing in the current aquaculture systems. BFT aquaculture systems always have high biomass, including cultured fish and microorganisms aggregated in bioflocs. The relative abundance of the top five phyla of bacteria did not differ significantly between the treatments (>0.05), however, a significant difference was observed at the genus level (<0.05). African catfish survival rates ranged from (91.11±1.53)% in LF, and (94.44±2.08)% in HF treatments. The food conversion ratio was (1.41±0.18) for LF and (1.27±0.26) for HF, and the specific growth rates were (2.13±0.04)%/d and (2.19±0.08)%/d, respectively. The efficiency of nitrogen use in food was 72.17% for LF and 71.34% for HF. It should be noted that the specific growth rates in the current study were lower than previous reports, perhaps owing to the uncontrolled water temperature and the extremely high suspended solids load. It is also worth noting that every time solids were removed, the rate of accumulation of TAN and NO₂^--N increased, and subsequently the NO₃^--N concentration decreased. The ratios of dissolved organic carbon (DOC) to total nitrogen (DOC/TN), or DOC to the sum of TAN, NO₂^--N, and NO₃^--N, in the two treatments were much lower than 20, which is the suggested value for biofloc aquaculture systems. This suggests that there is good control of TAN and NO₂^--N concentrations without any external organic carbon. The nitrification process was supposed to be ongoing in the current bioflocs systems. The results of the current study may lead to an effective water quality control system for culturing catfish, which may be applied in the commercial aquaculture industry.