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Cryptocaryoniasis remains a major parasitic disease and economic challenge for marine aquaculture. Cryptocaryoniasis in marine fish is caused by Cryptocaryon irritans (Ciliata). A theront is a motile, free‐swimming stage in the life cycle of C. irritans, which is typically the infective stage that actively seeks out a host to initiate infection. Population density and growth rate of theronts were investigated in Nile tilapia, Oreochromis niloticus fed with citric acid‐supplemented feed. The experiment involved feeding three diets with graded levels of citric acid (0, control diet, 0.5, 1 and 1.5 g kg−1 diet), to seawater‐adapted Nile tilapia (O. niloticus) juveniles for 21 days. The results showed that citric acid in the fish feed had an impact on the theront number of C. irritans in a manner of dose‐dependent. In the experimental cohort administered a diet supplemented with 1.5 g kg−1 citric acid, the population density of theronts was observed to be significantly reduced, measured at 29 ± 3.34, as opposed to 473.34 ± 16.48 in the control group at the culmination of the experiment. The observed population growth rate of theronts was significantly higher in the control group than in the group administered the citric acid feed (p <.005). The growth rate (r d−1) was 0.12 in control, 0.05 in 0.5 g kg−1, 0.031 in 1 g kg−1, and − 0.031 in 1.5 g kg−1 citric acid‐supplemented groups. Fish growth and feed conversion ratio were not affected by the citric acid in the feed. In conclusion, the findings of this investigation provide a valuable addition to our understanding of the potential protective effects of citric acid supplementation for fish against the C. irritans parasite. This is evidenced by the observed reduction in theronts present in the water. [ABSTRACT FROM AUTHOR]