A method of controlling fluid flow in a fluid network system by fluid machines includes obtaining a respective current flow rate associated with each fluid machine, obtaining a current fluid machine speed of each fluid machine, obtaining desired flow rates in the system, and determining a new fluid machine speed for each fluid machine based on the current fluid machine speed and a change in the fluid machine speed required to obtain the desired flow rates. The change in fluid machine speed is determined by minimizing a total fluid machine power which is a function dependent of the change in the fluid machine speed, the minimization being performed with constraints for flow rate, fluid machine pressure and fluid machine speed. The method include controlling the speed of the fluid machines according to the new fluid machine speeds such that the minimum total fluid machine power in the system is attained.