This study was carried out to evaluate the effects of humic acid (HA) on the nutrient removal efficiencies of aquatic duckweed plant (Lemna minor) from a water recirculating system used to culture Nile tilapia (Oreochromis niloticus) fish for 30days. The HA was added to water at three concentrations of 0 (Control), 1.5, and 3mg/L in triplicate. Water quality parameters, growth performance, and some hemato-biochemical parameters of the fish in variable HA concentrations were compared. The total ammonia nitrogen (TAN) and total phosphorous (TP) removal efficiency of L. minor increased with increasing the HA concentration from 0mg/L to 3mg/L (p<0.05). The concentration of nitrate (NO3) in the HA-3mg/L was higher than that in the other groups on days 20 and 30 of the fish cultivation period (p<0.05). The growth performance of fish improved in the HA-3mg/L compared to the other groups. The addition of different concentrations of HA to water had no adverse effect on the hematological properties of the Nile tilapia. The plasma aspartate aminotransferase (AST), alanine aminotransferase (ALT), and alkaline phosphatase (ALP) levels in the HA-0mg/L and HA-1.5mg/L groups were higher than in the HA-3mg/L (p<0.05). No significant differences in the plasma glucose and cholesterol levels were observed between the HAgroups (p>0.05), while the triglyceride level increased in the HA-3mg/L compared to the control (p<0.05). These results indicated that adding HA to water could be an effective method to enhance the bioremediation performance of the aquatic duckweed plants as biofilter and thus improve water quality, subsequently, fish growth performance in RASs.

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