This paper is about lead-lag robot fish swimming control by image processing. In robot fish aquariums, fish can generally move randomly in any path. This irregular movement may cause collisions between the fish, damaging them. Hence, an effective swimming control method is necessary. We thus proposed a lead-lag swimming control system for robot fish. Here, we simply study the detection of moving objects in an aquarium because we need to find the positions of moving robot fish. The locations of the robot fish are recognized using an image processing technique employing image and position sensors. This method is used to obtain the velocity for each pixel in an image and assumes a constant velocity in each video frame to obtain the positions of robot fish by comparing consecutive video frames. By using the position data, we compute the distance between robot fish and determine which robot fish is the lead fish and which fish is the lagging fish. The lead fish then waits for the lagging fish to catch up. The results of this proposed system are satisfactory in preventing collisions between robot fish. This system is exhibited in Busan Science Museum in South Korea.