In this paper a multirobot system consisting of small size ball-shaped mobile underwater robots is introduced. Robots form a cooperative society operating together for a common goal. This is made possible by inter-member communication and control architecture allowing cooperation. The test environment is a closed aquatic process containing tanks, pipes, and a jet pump. The task considered is cleaning of biologically contaminated spots in the process. Detailed hardware structure of a robot-member as well as the control architecture are introduced. Behaviour of the cooperative system is demonstrated in a test environment where contamination caused by biological algae growth is emulated by infrared panels behaving like a living biomass.