This paper is concerned with the problem of motion planning for multiple robotic fish swimming together to achieve a disk-pushing task. Based on our previous successful work on the design and development of robotic fish prototypes, we attempt further to investigate how to implement cooperation in groups of these fish. The motivation of this work is that the capability of a single fish robot is always limited while there are many complex missions in practice which should be accomplished by effective cooperation of multiple fish. A disk-pushing task is employed as an example to demonstrate the design of cooperative behaviors as well as strategies. Corresponding experimental results show that higher efficiency and much greater capabilities can be obtained when these fish planning their motion properly.