Well water is your best bet, I think. But I've heard that well water is devoid of O2, so you are correct in that you have to 'treat' the water.
Splashing the water over several obstacles on it's downward path to the pond works, but there is a trade off to the well water getting warmer along the way as it splashes down and mixes with the air.
You have to add the O2 to the water, though.
