The potential distribution of tropical fish species in Eastern Europe—Gambusia holbrooki (introduced for biological control) and Poecilia reticulata (aquarium species, found in waste waters of big cities)—tend to be of particular interest in terms of global climate change. After GIS modeling of our own data and findings listed in the GBIF databases (2278 points for G. holbrooki and 1410 points for P. reticulata) using the Maxent package and ‘ntbox’ package in R, 18 uncorrelated variables of 35 Bioclim climatic parameters from CliMond dataset, it was found out that by 2090 guppies will appear in the south of Ukraine (Danube river’s estuary, as well as in several places in the Caucasus and Turkey with habitat suitability > 0.3–0.5). G. holbrooki will also slightly expand its range in Europe. Limiting factors for G. holbrooki distribution are: bio1 (Annual mean temperature, optimum +12–+24 °C) and bio19 (Precipitation of coldest quarter (mm). Limiting factors for P. reticulata are: bio1 (optimum +14–+28 °C), bio4 (Temperature seasonality), bio3 (Isothermality). Unlike G. holbrooki, guppies prefer warmer waters. Such thermophilic fish species do not compete with the native ichthyofauna, but they can occupy niches in anthropogenically transformed habitats, playing an important role as agents of biological control.