Regarding the mercury switch I referenced: These switches are meant to control pumps, as where you want a pump to turn on or off with regards to water level. The important consideration is "pump up" or pump down" you have to have the right one for what you want to do. Jerry, on mine, the plug is the only connection. You plug the mercury switch in the receptacle, then you plug your pump into the back of the mercury switch plug.
Now, if what you really want to do is ADD water to your pond to compensate for evaporation, a cheaper alternative is at hand. First, get an Orbit water control solonoid (Around $12-$15) like those used in lawn irrigation systems. Pipe it into your water supply with the flow going to your pond. Next, you need a power supply for the solonoid. For that you can use any 15-24 volt AC power supply. See the 18 volt AC/1 amp wall charger second item down on the left here:
http://www.surplussales.com/powersupplies/PowerS-18.html You may even have one in your junk box that you can use, just be sure to read the label.
Next you will need the switch, which in this case is a "liquid level" switch. You convert it from Normally closed to normally open simply by inverting it. The mini switch is $20.00 from aquatic eco systems. Mount the switch in the pond water at the level you want and wire it in series between the power supply and the solonoid. You could use a piece of flat 1/8 inch thick by 1/4 inch wide aluminum stock. Drill a hole in it to accomodate the switch and mount the metal strap under a bridge, etc. Adjust the desired water level by bending the metal strap up or down. Water goes down, water turns on. Water goes up, water shuts off. Just don't put the water inlet close to the switch to prevent "wave" cycling.
http://www.aquaticeco.com/subcategories/1213/Water-Level-Switch/liquid%20level%20switch/0
