The aim of this study was to assess the impact of major parasitic crustacean invasions on farmed marine fish in Ismailia Province, Suez Canal Region, Egypt. A total of 540 premature, 180 Dicentrarchus labrax (225 ± 25 g), 180 Sparus aurata (150 ± 25 g) and 180 Mugil cephalus (125 ± 25 g) were collected from assimilatory ponds in Ismailia farms. They were investigated for crustacean infestation between May 2021 and August 2022. Infected fish were characterized by respiratory symptoms, reduced food intake and growth rate, abrasion, erosion, and ulcers of varying sizes. The overall prevalence in untreated premature animals studied was 53.44%, D. labrax 63.87%, S. aurata 53.60%, and M. cephalus 36.92%, respectively. While, the prevailing of infestations after treating the same fish with 5% of microalgae (Amphora spp and Nanochloropsis spp ) that added to feed were 21.48 % (33.88, 23.61, and 16.94% for D. labrax, S. aurata, and M. cephalus, respectively). The parasite species detected were the crustaceans Lernanthropus spp. and Caligus spp. from D. labrax, M. cephalus, and S. aurata. Histopathological alterations were recorded in the gills and musculature. Microalgae played an important role in the spread of fish diseases in the studied fish.