In the wild, both food availability and an animal's requirements may fluctuate. Given the choice to feed on different diets, animals may exhibit preferences in their selection. Although macronutrient selection has been investigated in mammals and birds, few attempts have been made to investigate dietary selection in fish. In the present study we investigate the feeding of goldfish on three "pure" macronutrient diets. Twelve single goldfish and one group of four goldfish, initially weighing 163.1 +/- 7.4 and 70.2 +/- 8.0 g, respectively, were maintained in 54-liter tanks at 20 degrees C under a 12 h:12 h light-dark (LD) cycle. Three pelleted diets were made containing 50% of each macronutrient: protein (casein + gelatin), fat (fish oil + soy oil), or carbohydrate (dextrin). All the diets included mixtures of vitamins and minerals, a binder (sodium alginate), and cellulose as a filler. During trials, three self-feeding devices delivering the protein, fat, or carbohydrate diet were installed in each aquarium. Goldfish soon learned to select from among the three diets. Overall daily food and digestible energy demands differed significantly between macronutrients. The demand for protein was lower than that for carbohydrate (16.6 and 33.7 g/kg BW/day, respectively), and the digestible energy demand for protein was lower than for fat (108.6 and 308.5 kJ/kg BW/day, respectively). Goldfish made their dietary selections on the basis of energy content, so that food demand increased to compensate for changes in the digestible energy density of the diets. Correlation analysis revealed a significant positive relationship between carbohydrate and fat demand (n = 8), whereas there were significant negative (n = 3) and positive (n = 1) relationships between protein and fat. Different daily patterns for the selection of each macronutrient were detected in some fish (i.e., carbohydrate during daytime, protein at the beginning of the night, and fat during the transition phase). In short, our results demonstrated the ability of goldfish to compose their own diet from three basic macronutrients, highlighting the complex pattern of behavior in their selection. Therefore, these new findings should be considered when studying feeding rhythms and the design of diets for fish.