Several studies have shown the positive effect of phytase on phosphorus utilization by fish and animals, with the use of phytase sources determined for different fish species. Few studies have tested phytase response to different diets, which may affect nutrient availability for optimum growth due to differences in phytate location. The research, therefore, studied the effect of phytase to diets based on soya bean and groundnut meal for Clarias gariepinus on nutrient availability and growth. In trial 1, four groups of soya bean S1, S2, S3 and S4 replaced fish meal at 25%, 50%, 75% and 100% and supplemented with 250, 500, 750 and 1000 FTU/g phytase, respectively. In trial 2, four groups of groundnut meal diet G1, G2, G3, G4, G5 and G6, were similarly supplemented with the same phytase levels used in experiment 1. Fish meal control (S0=G0) was not supplemented with phytase. Result showed that 250 FTU/g phytase showed the highest mean weight gain for both plants. In conclusion, the research has shown that the chemical nature of phytate, rather than its Uzezi et al.; ARRB, 31(4): 1-21, 2019; Article no.ARRB.26092 2 concentration and location, may influence the utilization of phosphorus for optimum growth in the fish by supplementing 250 FTU/g, with a range of available phosphorus requirement of between 0.75% (Y = 0.363 + 4.155X -2.772X 2 , R 2 = 0.759) and 0.80% (Y = 0.307 + 3.303X -2.059X 2 , R 2 = 0.210) Original Research Article