Maybe something like a float valve on a toilet. Some septic systems have 2 floats in case of an overflow.

Hey Mark, a float switch like I use installing aerobic septic systems would be easy to rig up. You'd just need someway to fasten it to the inside of the tank, or rig up a pipe inside to fasten it to- in the septics the dangle from the irrigation pump plumbing at different heights to trigger the pump and high water alarms. They are usually wire inline on the power supply but could be rigged up to operate a relay.

Something like these:

http://www.septicsolutions.net/store/floatswitches.htm

The thing with floater valves is that they can freeze of get stuck.

I've used ultrasonic sensors in liquid storage tanks. Here is an example for measuring the water level in a tank controlled using an arduino board. This could easily be adapted to switch a pump with a relay.

An even simpler solution uses three wires and a relay. The first wire goes to the bottom of the tank, it carries a low voltage. The second wire goes to the lowest water level, and the third to the top water level. When the circuit to the second wire is broken, the water level is too low, and the relay switches on the pump. When the circuit to the third wire is closed the upper water level is reached and the relay witches the pump off.