Fingerlings of hybrid tilapia (Oreochromis niloticus x Oreochromis aureus) were cultured in 18 glass aquaria (75 x 40 x 60 cm). Two species of phytoplankton (Chlorella  vulgaris & Scenedesmus spp) were used. The experiment included six treatments; each in three replicate aquaria. The 1^st treatment (Urea) was fertilization by urea at the rate of 435 mg commercial urea / aquarium. The 2^nd treatment (Urea+phy.) was the same rate of urea plus the addition of phytoplankton at a rate of 5X10⁴ cells/ml of water. The 3^rd treatment (MSP) was fertilization by mono superphosphate (MSP) at a rate of 830 mg commercial MSP / aquarium. The 4^th treatment (MSP+phy.) was the same rate of MSP plus the addition of phytoplankton at a rate of  5X10⁴ cells/ml. The 5^th treatment (Urea+MSP) was fertilization with urea and MSP mixture at the rates mentioned above for each fertilizer. The 6^th treatment was the Control; without any fertilization or phytoplankton application. Phytoplankton played an important role in removing ammonia and nitrite from the water milieu. The average of individual fish body weight (ABW) at the end of the experiment was (29.37 ± 0.60 g) in the Urea+phy. treatment which was significantly (P<0.05) greater than that of the Control (25.22 ± 0.84 g), and the other treatments. The fish in Urea treatment had the lowest value of ABW (19.22 ± 1.30 g), this is ascribed to the elevated values of un-ionized ammonia (NH₃) and nitrite concentrations in the water of this treatment. On the contrary Urea+phy. treatment had the lowest values of un-ionized ammonia and nitrite due to uptake of the nitrogenous compounds (urea in this study) from the water milieu by phytoplankton, consequently, the  medium become more appropriate for fish growth.