Carp ponds are inhabited mainly by aquaculture fishes but also harbor alien unwanted species that may threaten native fish assemblages when escape from the ponds. In the current study fish communities of inflow and outflow watercourses (canals) of the fish ponds in two typical common carp fish farms were studied. The results revealed that outflow canals contained cultured fish that escaped from the ponds. Invasive alien species (IAS) were also observed e.g. stone moroko (Pseudorasbora parva) and Prussian carp (Carasus gibelio). On the other hand, the canals harbored valuable endangered fish e.g. common nase (Chondrostoma nasus) or brown trout (Salmo trutta) and protected species stone loach (Barbatula barbatula). We have shown that fish ponds affect neighboring watercourses, and that the canals themselves can contribute to the species richness of ichthyofauna. Therefore, monitoring of these ecosystems should be carried out on a regular basis.