The effects of feed, fertilizer and combination of fertilizer and feed were investigated on water quality parameters, natural food production and growth rate of Nile tilapia (Oreochromis niloticus). The experiment was conducted for six months in nine concrete tanks, each with an average surface area of 3.4 m2. Three tanks were subjected to a treatment of full amount of the required formulated feed (T1), three tanks under fertilizer application only (T2) and three tanks under a combination of half the amount of the required feed plus fertilizer application (T3). Three grams of urea and two grams of Di-Ammonium Phosphate (DAP) fertilizers per m2 were applied once per week for treatments T2 and T3. Sex-reversed Nile tilapia with average weight of 0.9 g were stocked at 3 fish /m2. Fish were fed twice per day with a formulated diet containing 30% CP for treatments T1 and T3. Algae were scrubed from nets for quality and quantity analysis every month and fish were sampled twice per month for weight measurement. Water quality parameters were measured once per week.
Our results indicated that dissolved oxygen values were lower (p < 0.05) in the treatment for feed only (6.51 ± 0.20 mg/l) and higher in the treatments for fertilizer only (9.97 ± 0.21 mg/l). pH and temperature did not differ (p ˃ 0.05) among the treatments. Significantly higher (p < 0.05) fish growth rates were observed in the treatments with the combination of feed and fertilizer (1.6 ± 0.03 g/d) and feed only (1.4 ± 0.02 g/d) than in the treatment with fertilization alone (0.2 ± 0.0 g/d). Although growth rate between the treatments for feed and combination of feed and fertilizer did not differ significantly, the feed conversion ratio was lower (p < 0.05) for the combination of feed and fertilizer (1.5 ± 0.0) than for feed only (3.3 ± 0.2). High growth rate of fish under the combination treatment may be contributed by the availability of both artificial and natural food of high quality due to the fact that both feed (27.55 ± 0.19% CP) and combination (25.23 ± 0.2% CP) had higher (p < 0.05) algal Crude protein and organic matter contents compared to the fish cultured under fertilizer alone (13.74 ± 0.06% CP). Primary productivity was lower for the fertilizer treatment (0.14 ± 0.04 mg C/l/h) compared to the feed (0.50 ± 0.05 mg C/l/h) and combination (0.51 ± 0.15 mg C/l/h) treatments. Thus, we can conclude that the use of combination of half feeding and fertilizer application increased gross primary productivity, algal crude protein content and fish growth rate compared to fertilization only or feeding only.