This paper presents a temporary multimedia installation set up at the Civic Aquarium of Milan. Thanks to four web cameras located in front of the tropical fishpond, fish are tracked and their movements are used to control a number of music-related parameters in real time. In order to process multiple video streams, the open-source programming language Processing has been employed. Then, the sonification is implemented by a Pure Data patch. The communication among the parts of the system has been realized through Open Sound Control (OSC) messages. This paper describes the key concepts, the musical idea, the design phase and the implementation of this installation, discussing also the major critical aspects.