In this experiment, goldfish were able to solve a four-arm maze after a week in order to obtain food. The fish were able to navigate the whole maze by memory and also from working out which way to go from separate visual clues remembered previously. Thus the fish were able to build a complex representation of their worlds.
"Goldfish were trained to obtain food in a four-arm maze placed in a room with relevant spatial cues. ... all groups were able to solve the different tasks with high accuracy after 1 week of training ... goldfish are able to solve spatial tasks ... and to build complex spatial cognitive representations of their environment."