At farm stores you can get mechanical float valves for cattle tanks.
We have used them in 1500 gallon plastic storage tanks that we use as surge tanks to fill our ag sprayer out of. We removed the lid of the tank and installed one of these valves on the lid at the desired water level (full but not so full as to run out the lid). The water flows through the valve till the tank is full and the water is cut off. Once we fill out of the tank and the water drops the valve allows the tank to refill. Standard garden hose screws into the valve.
That way no one has to babysit the tanks while they are filling. We have used a couple different types of these valves and generally speaking the better ones that cost a little more are more dependable and last longer. Farm stores have them in stock around here.
Depending on your flow needs, a bathroom stool tank float valve might also work.
Edit: I misunderstood the question. I thought he already had a working water system and only needed to control the water level in an open tank. I see now he was wanting to switch a pump on and off.
Last edited by snrub; 12/19/13 08:07 AM. Reason: misunderstood question so gave poor answer
