The formation of stable periphyton communities is undesirable for all types of hydrotechnical systems, especially functioning watercooling systems. To combat the development of biological fouling in the cooling ponds of nuclear power plants bioreclamation species are widely used, which include Mozambican tilapia. It was unintentionally brought into the cooling reservoir of the Novovoronezh nuclear power plant; however, it formed a stable and numerous population there. The food base for the fi sh population in the cooling pond is depleted due to diffi cult (sometimes extreme) environmental conditions, which can lead to competition for food items. Under these conditions, tilapia switches to feeding on bacterial-algal mats, which develop abundantly in the warm-water part of the reservoir. In areas eaten by tilapia, the biomass of fouling is reduced by one and a half times and the content of mineral substances decreases. The study of the contents of the intestines of tilapia made it possible to confi rm these observations, since representatives of the phytoperiphyton were found in the food bolus: cyanoprokaryotes, diatoms and green algae, for which even a taxonomic affi liation was established. The results of the research made it possible to clarify the mechanism o f integration of this southern species into the ichthyocenosis of the reservoir and its positive role in reducing biofouling.