I've started a small project to control my pond pump and air pump. The goal is to let a Arduino control the pump.
Stop when the water level gets too low
control pump time according to the temperature
switch on the air pump when the temperature gets to high or low
It has features which may interest others: switching AC and detecting water level. The project is far from complete but i will work on it when i find the time.
An alternative for liquid sensing is to use two vertical, parallel, insulated strips of metals separated by a small air gap. Clearly this is a capacitor. As the liquid rises the air gap is replaced with water gap which has a different dielectric constant and changes the capacitance. Either charge and discharge the capacitor through a resistor (probably very high, this is a small capacitor) and measure the voltage on either end of the cycle, or maybe better, make an oscilator's frequency controlled by the capacitance and count pulses.