Background and Aim: Inadequate quality of water and the presence of contaminants such as silver nitrate in water can cause stress in fish and endanger their life by reducing the immune system function. The aim of this study was to determine the effect of different prebiotic levels of oyster mushroom (Pleurotus ostreatus) on some immune parameters of tilapia serum exposed to silver nitrate toxin.
Methods: 120 tilapia were treated for 42 days in 4 groups with 3 replications (n=10). These groups were 1) control, no oyster mushroom prebiotics, 2) food containing 0.05%, 3) food containing 0.1% and 4) food containing 0.2% prebiotic oyster mushrooms. Silver nitrate toxin with a concentration of 0.5 ppm was added to each group for 16 days. Finally, the biochemical parameters of fish in serum were measured.
Results: Serum biochemical parameters in the control group were significantly different from the groups fed with oyster mushrooms (0.05, 0.1 and 0.2% oyster mushroom prebiotics) that were exposed to silver nitrate toxin (P<0.05). The use of oyster mushrooms prebiotics had a positive effect on serum biochemical parameters; more effects were observed on AST, albumin, glucose and total protein levels.  
Conclusion: This study showed that the level of 0.2% prebiotic oyster mushroom can partially neutralize the effect of silver nitrate toxin in tilapia.