The exposure of guppy fish (Poecilia reticulata), infected with the monogenea Gyrodactylus turnbulli and Dactylogyrus sp. to humic-rich culture water and feed, reduced both the infection prevalence (% of infected fish) and the infection intensity (parasites per fish) of the two parasites. Specifically, among fish exposed to: (a) humic-rich water and sludge from a recirculating system (RAS) and (b) synthetic humic acid (HA), infection prevalences of Gyrodactylus turnbulli were 17% and 25% respectively, as compared with an infection prevalence of 52% in the control group. The lower infection prevalence was accompanied by a significant reduction in the infection intensity: from 3.8 in the control group to 0.2 and 0.3 parasites per fish in the RAS and HA treated fish respectively. The infection prevalence and intensity of Dactylogyrus sp. were significantly lower (infection prevalence: 2.5%; infection intensity: 0.3 parasites/fish) in guppies exposed to RAS water and sludge than in the control group (infection prevalence: 50%; infection intensity: 0.8 parasites/fish).