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The current study aimed to investigate the most prevalent ectoparasites among some Egyptian tilapia hatcheries and find out the appropriate therapeutic and prophylactic doses. For that, 500 Nile tilapia fry weighing 0.02 g were collected from several hatcheries at Sharkia Governorate, Egypt, over the period of April to September 2017, examined for the presence of trichodinids and monogeneans. The prevalence and intensity of such parasites were recorded. LC50 of the five selected chemicals was examined against 1000 healthy fry for 24 h. The therapeutic and prophylactic doses were evaluated against 360 heavily infested fry and 600 newly hatched ones for 72 h and 3 weeks of indefinite treatment, respectively. Infested fish exhibited erratic swimming, detached skins, and hemorrhagic gills. The most common parasites identified were Trichodina fultoni, T. salmincola, Cichlidogyrus sp., and Cichlidogyrus tubicirrus. The LC50 was 81.31, 235.57, 7.19, and 15.48 ppm for formalin, hydrogen peroxide, potassium permanganate, and methylene blue, respectively. All chemicals were effective in treating trichodinids, while formalin was the only efficient chemical to treat monogeneans 72 h post-exposure. Potassium permanganate showed the best preventive activity against both types of parasites for successive 1 week post-exposure. This study highlighted the optimal use of chemical drugs for the control of ectoparasites from an economic perspective. [ABSTRACT FROM AUTHOR]