Author(s): MM El-Moghazy and AM El-Morsy

Abstract: In the present study, growth performance of Nile tilapia (O. niloticus) was evaluated. Nile tilapia were divided into four groups in two replications according to time of water change, after two days (control), after four days (T1), after six days (T2) and after eight days (T3). The feed was supplied at the ratio of 3% of wet body weight of fish fingerlings twice a day for a period of 12 weeks. Final body weight gain and final body length values of (T1) were significantly (P<0.05) higher than other treatments. Also, the values of the mean growth rate of (T1) were significantly (P<0.05) higher than other treatments. Final body depth values of (T1) were non-significantly (P>0.05) different with control, but it was significantly higher than (T2) and (T3) (P<0.05). The specific growth rate (SGR) values of (T1) were significantly higher than other groups (P<0.05). The best growth performance parameters of Nile tilapia were in (T1), which decreased water consumption too, decreased the cost of fish production by decrease the fuel consuming without affective on growth performance. During the first six weeks the log of total bacterial count in the water increased slightly by increasing the time in all fish groups (C, T1, T2 and T3). By the next weeks (W8, W10 and W12) log of total bacterial count in the water increased markedly until reaching the maximum values at the end of experiment after 12 weeks being 5.3, 5.28, 5.46 and 5.60 Log CFU/ml in C, T1, T2 and T3, respectively. Generally, the values of total bacterial count in water took the lowest values in C followed by T1 and T3 while T3 was the highest values. The BOD5 in C ranged between 4.9 to 8.6 mgO2/L. The lowest value of BOD5 in T1 was 5.5 mgO2/L and the highest value was 8.2 mgO2/L. It was observed that the values of BOD5 in T2 were fluctuating between 5.7 and 7.9 mgO2/L. The values of BOD5 in T3 were 6.3, 6.9, 7.2, 7.3, 7.8, 8.2 and 9.2 in W0, W2, W4, W6, W8, W10 and W12, respectively. The values of the correlation coefficient between the log of total bacterial count in water tanks of Nile tilapia during the experimental period and the biological oxygen demand were 0.678, 0.869, 0.879 and 0.896 in C, T1, T2 and T3, respectively, these values occurred between 0.6 and 1.0 and that means the correlation is strong.

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