Thanks to everyone for the suggestions. Some people are misunderstanding the reason why I want a level gauge. I don't want the gauge so that I know when the tank is full, I also don't need it so that I know when to start the motor.
I don't have a motor installed, since the water pressure is good in my locality, so water goes straight up to the overhead water tank without needing any motor. On the ground floor I have a 3 stage filter installed on the water main. The filter ensures that the water in the tank stays clean. The reason I have installed a filter is that the main water tank in my locality is cleaned once a month and when they clean the tank, it sends all sorts of silt into the water coming to our home, which ends up sticking in the non-return valves which basically kills them. Two cartridges of the filter are for filtering out large particles, the third stage filter is a very fine 1 micron filter, which basically needs to be replaced every 15 days.
But sometimes it gets choked before 15 days, which stops the water from going to the overhead tank. When that happens, the water tank starts to empty and suddenly there is no water in the taps. Then I realize that the filter needs to be replaced.
Now if I had some way of knowing how much water is in the tank, I would be able to tell if the filter is still good or whether it needs to be replaced.
The device below seems to be good, but I was wondering if something like this can be wired to a gauge.
Water Level Advance : General Purpose : Kits : EVE-eVision Electronics Pakistan : Microcontroller Development Boards