I bought a really cheap ($20) pond pump and tried it out yesterday. I cooled my wort to 90F running garden hose water through my chiller, then changed it over to my pond pump. I had the pump in the bottom of my mash tun with the false bottom removed. I had approx a gallon of water in it and 6lbs of ice. It took 10 minutes to get the water from 90 to 70. The ice was gone but the water was still really cold. I dumped in a second 6lb bag of ice. 10 minutes later my wort was 62 degrees. My ground water was 72 so without the pump I never would have gotten the wort under 72 and it would have taken me a long time to get it to that temperature.