This study assessed the effectiveness of essential oils (EO) from cinnamon, garlic, and citronella mixed with feed pellets to reduce marine leech infestations in hybrid groupers. The trial involved 3,000 groupers fed a 14-day diet with specific EO, while another 1,000 received standard pellets before being placed in infested floating cages. Fish were divided into four groups, each with 1,000 individuals. Results showed that the group on standard pellets experienced a 100% infection rate and a mean intensity of 5.9 leeches. In contrast, the EO-treated groups showed significantly lower prevalence, with infections ranging from 53.3% to 75% and mean intensities between 1.1 and 2.4 during the first four weeks. The cinnamon EO group had the fewest leeches attached, demonstrating a notable advantage over citronella and garlic. These findings suggest incorporating EO into diets can effectively mitigate marine leech infestations, offering a viable strategy for protecting hybrid grouper during critical periods.