In this paper, we introduce an algorithm for locating sound-producing fish in a small rectangular tank that can be used, e.g., in behavioral bioacoustical studies to determine which fish in a group is sound-producing. The technique consists in locating a single sound source in the tank using signals gathered by four hydrophones placed in the tank together with a group of fish under study. The localization algorithm used in this paper is based on a ratio of two spectra ratios: the spectra ratio between the sound pressure measured by hydrophones at two locations and the spectra ratio between the theoretical Green's functions at the same locations. The results are compared to a localization based on image processing technique and with video recordings acquired synchronously with the acoustic recordings.