In West Hungary, north of Lake Balaton, we find a lake that is completely unique. The world's largest biologically active, medicinal thermal lake, Lake Hévíz. Its name is familiar to many people, both in Hungary and internationally. Its gentle, warm waters are used for medicinal treatments, and the spa is open to visitors all year round. But Lake Hévíz also has a hidden world. Today, this is the setting for a strange but completely unsuspected ecological experiment. Both the lake and the outflow channel, known as the Hévíz Stream, have become home to tropical fish species. Hidden among the Indian water lilies, Central American cichlids, known from pet shops, live their daily lives. Huge swarms of firehead cichlids forage on the bottom, jewel cichlids protect their young in the cover of aquatic plants, and predatory jaguar cichlids hunt unsuspecting small fish. But how did this diverse yet alien community come to be? And what are the consequences of their presence for native wildlife?