Plant phytochemicals have been known to boost the immunity of fish with the release of electrolytes in the cultured water body. This study evaluates the phytochemicals present in the neem plant that can acts as anti-water fouling and anti-stress for culturing fingerlings of African Catfish Clarias gariepinus . Fresh matured leaves of Azadirachta . indica (neem) were collected, air dried and aqueous-extract was used to screen for some phytochemical presence. Two hundred (200) fingerlings of Clarias gariepinus were purchased from LASU hatchery and stocked in a 500 litres tank for 3 weeks (21days) in order to induce stress on the fishes by overstocking, water was not changed and feeding was not regular and fed once per day. The leaf extracts were analyzed at the World Bank Step B Laboratory of the Federal Institute of Industrial Research, Oshodi, (FIIRO), Lagos State using the Atomic Absorption Spectrophotometer (SCHIMAZU-AA-7000). Five (5) grams of the air-dried and finely pulverized sample was weighed into a 100 ml of digestion flask. For proximate composition, moisture content was determined using the hot air oven, by drying the sample at 105 °C ± 2 °C until a constant weight was obtained. Total lipid was determined by Bligh and Dyer method using chloroform/methanol (1/1, v/v). Crude protein content was determined by Kjeldahl’s method (Nx6.25). Ash content was determined after combustion. The result revealed the moisture (5.67±0.35%), Carbohydrate (82.15±0.92%), Protein (1.52±0.06%), Ash (2.96±0.03%), fibre (5.04±0.0.11%) and fat content (2.65±0.34%). The mean Temperature values observed was (27.92±1.24) mg/ L in the control tank (tank 1) and (26.95± 1.34 ) mg/L in the tank with neem leaf (tank 2). The dissolved oxygen recorded was (5.70± 1.23) mg/ L in tank 1 and (3.82±0.98) in tank 2. The pH value ranged from (6.65±1.08) mg/ L (minimum value), which was observed in tank 1 while the highest value of (7.66±1.17) mg/ L obtained in tank 2. All other water quality parameters measured such as Total suspended solids, CO2, and Ammonia were within the permissible limit as stated by WHO/USEPA. The mineral content had Calcium, Magnesium, Potassium, Sodium, Iron, Zinc, Lead, copper, Manganese and Nickel. Magnesium (2019.125±11.349) recorded the highest value, while Nickel (0.033±0.297) was recorded with the lowest value, Ninety-eight (98) chemical compounds were obtained in Azadirachta indica. Some of the chemical compounds include 2,2-Dimethyl-1,3-dichloro-1-cyano-4 (trichloromethyl)cyclobutane, Tocopherol, Octadecanoic acid methyl ester, Propanoic acid 2-oxo- methyl ester, Cyclopentanol,5,7-dimethyl-6,8-dioxabicyclo[3.2.1]octane, 3,5,5-trimethyl-4-Amino-6-hydroxypyrimidine , Hexadecanoic acid methyl ester and n-Hexadecanoic acid In conclusion, neem leaf were able to reduce water fouling within thirty days as well boost fish immunity with low fish mortality when fish were stressed.