The water level in the tank, or other container can be measured in different ways greater or lesser complexity. The simplest diagram of the control device shown in figure.
Once the water level falls below the electrodes begins to glow led. When water levels are high field-effect transistor almost never conducts current, because the shutter is associated with the earth and there is no potential difference between the gate and source.
When the water level drops, the connection gate-source disappears. On the shutter through the resistor R2, there is a positive potential and the field-effect transistor is in the open position. Led lights.
If you want the led light when the electrodes are shorted by water, then you need to connect the "ground" electrode in the circuit to the positive and the resistor R2 between the gate and the source.