To control the liquid level in the draining tank three equipment changes are necessary. 

First, we have to measure the level, and we have selected a pressure difference sensor for this purpose. 

Second, a control valve is needed to provide a variable resistance in the outlet pipe from the tank.  This is one possible configuration to control this self-regulatory system.

Third, a calculating device is needed to perform the controller calculation.  This would normally be a digital processor with communication with the sensor and valve.

Note: A pump is not required unless the liquid head in the tank is not sufficient to provide the desired flow rate.