Three dosage levels of black seed, garlic and commercial Biogen were administered for three months in summer and six months in winter to test their stimulation effect on growth, survival and response to challenge infection in Oreochromis niloticus. At the end of summer, no significant difference was observed in the growth performance of O. niloticus with most treatments, but the level of non-specific mortalities was reduced in all treatment groups compared to the control group. Significant changes were seen in nitroblue tetrazolium (NBT) values in groups treated with 1.0 and 3.0% garlic. Mortalities following challenge with Pseudomonas fluorescens were lower in the groups that received garlic compared to the other two treatments. At the end of winter a significant increase in body weight gain was seen in fish fed a 1% garlic-enriched diet. Significant changes were also noted in condition factor in fish fed with 2% garlic and 1.5% Biogen. Non-specific mortality was reduced in all groups that received immunostimulants, except Biogen. Immunostimulants improved the general health of the fish, enabling them to overcome stress due to cold during winter, thereby improving growth.