The simplest way is like I explained before, using simple proportional control. However, this type of control will swing the output when the inflow drops by 500 LPS. The OP indicate the surface area of the tank, but did not indicate the volume of the tank, nor did he indicate the duration of the inflow drop. If the 500 LPS * length of time of reduces flow is less than the tank volume (or close), you may be able to use PID for the level control under 'normal' conditions, but when the 500 gpm flow reduction occurs, the tank level will drop. When this occurs, put the PID in a manual mode, with the output set to a speed a little lower than the output before the tank started dropping. Then after the 500 LPS excursion ends, the tank level will start to rise. At this point, put the PID back in auto. With proper tuning, it should bring the tank back to the set point.