The present study aimed to assess the effect of water hyacinth (Eichhornia crassipes) as a biofilter on water quality, growth performance and survival of koi carp (Cyprinus carpio) in an aquaponic system. Water hyacinth was used at three densities of 2 bushes, 4 bushes and 6 bushes. Each treatment contained 10 fish weighing 11.43±0.1 g (in 80 L aquarium) in 3 replicates, and the experiment was run at 22-24°C for 42 days. A control group was also included without the plant. The obtained results showed that there was no significant difference in dissolved oxygen (DO) level among the treatments while DO in all treatments were almost insignificantly higher than the control (p>0.05). Total dissolved solid in all treatments were significantly lower than control with the lowest level obtained in 4 bushes group (p<0.05). Total suspension solid level in all treatments were significantly higher than control (p<0.05). Also, PO₄ value was decreased with an increasing in the plant density. Both nitrite and nitrate in all treatments were lower than control group. Cadmium content of water was reduced with increasing in plant density. These values in all densities were lower than the control group. A better performance in final weight, weight gain, daily growth weight, specific growth rate, protein efficiency ratio and food conversion ratio were obtained in 4 bushes than the other treatments. Also, significantly lower mortality (about 12-34%) was observed in all treatments than the control fish (p<0.05). These data show that the combination of water hyacinth especially at 4 bushes density together with koi carp can improve water quality and increases fish production.