A method of feeding fish in a tank with water comprises the steps of forming in the fish tank a region for feeding fish, introducing into the region a water-stable fodder and forcedly moving the water in the fish tank so as to maintain the fodder floating in the feeding region. In an arrangement for performing the method means forming a feeding station, and means for forcedly moving water in the feeding station are provided. The method of the invention may also comprise the step of utilizing heat liberated during operation of a biological reactor for heating agitated water in the fish tank. A water-stable fish fodder comprises particulate nutrient matter and a protein-containing binder imparting substantial water stability to the fodder formed as a pallet.