Effects of different carbon to nitrogen ratios in the biofloc system with sugarcane
molasses as organic carbon source on water quality, growth performance, and body
composition of common carp (Cyprinus carpio L.) fingerlings were investigated in a
90-day trial. A total of 450 fish (14.17 ± 0.36 g) were fed twice daily with commercial
diet at 5% of their body weight under a 12:12 light:dark regime and cultured in 1
control (without extra carbon source) and 4 experimental treatments (C/N: 11:1, 15:1,
19:1, and 23:1) with 3 replicates at a density of 1.06 kg m−3 per tank. Based on the
results, the water electrical conductivity (EC) and total dissolved solids (TDS)
(measured daily), flocs volume (FV), and total suspended solids (TSS) (measured
once a week) were increased from C/N 11:1 to C/N 23:1. The variation patterns of the
average levels of total ammonia nitrogen, nitrite, and nitrate (measured once a week)
were similar and showed a decreasing trend with an increase in C/N ratio. Total
heterotrophic bacteria was also positively affected by increasing C/N to 23:1. The
highest fish final weight (59.36 ± 0.24 g), weight gain (45.21 ± 0.47 g), and protein
efficiency ratio (PER, 1.77 ± 0.04) were recorded in C/N 19:1. The lowest (1.46 ±
0.03) and highest (1.51 ± 0.05) feed conversion ratio (FCR) and the highest (1.60 ±
0.03%/day) and lowest (1.49 ± 0.01%/day) specific growth rate (SGR) were observed
in C/N 19:1 and the control group, respectively (P < 0.05). By increasing the C/N to
23:1, FCR increased and the feed intake, final weight, weight gain, PER, and SGR
decreased. The highest crude protein content (77.31 ± 0.43 %DW) and the lowest
crude lipid content (12.13 ± 0.32 %DW) of the fish carcass were observed in C/N 19:1
while these values for flocs were not significantly affected by C/N ratio. Overall, the
biofloc system with C/N 19:1 improved the water quality and growth performance of
common carp while not negatively affecting the carcass proximate analysis.