Aquaculture has become an important sector of animal food production thanks to a fast and continuous growth over the last few decades. Both marine and fresh water fish can be reared in fish farms, and there are species that are mainly produced by aquaculture rather than capture. As most of the reared fish species are sensible to water temperature and its fluctuations, having the control of this parameter in fish farms becomes an important issue that has to be taken into account. In this paper, three different fish farms are studied by simulating the water temperature when a thermal energy storage (TES) system using phase change materials (PCMs) is implemented. The effect and the estimated amounts of PCM required in each particular case are also analyzed. The results show that appreciable effects can be obtained, especially when the TES unit is used in combination with auxiliary sources of energy, such as solar collectors.