When, according to the invention, constructing a drive system for rotating a cleaning head in a tank and allowing the system to comprise a hysteresis coupling having two magnetic parts, one being present inside the liquid space (4) and the other outside the liquid space, respectively, a completely liquid-tight separation between the housing (2) and a gear (14) may be ensured. The magnetic part (8, 18) of the hysteresis coupling may be formed by electromagnets, and with a displaceable (13) position of one part relative to the other part (11,19), the coupling may be adjusted with respect to the transferred torque and/or speed.