Vithal,

If you have clearnace at the top and bottom of the tank, consider using a float switch similar to what is used on sump pumps.

A float switch is likely to be less expensive than ultrasonic methods. An optical method usually necessitates periodic cleaning of the detector window.

A sealed switch can be obtained from Grainger or McMaster-Carr. Choosing an appropriate material and size for the float is probably the most difficult problem to solve.