I often get asked how big a pump has to be. I generally use as big a pump as I can afford to get maximum sound from the waterfall without splashing water out. But all you need is a pump big enough to circulate the water once per hour. And just a reminder, water can fall 1/2 the distance of the width of the water it falls into. If the width is too small or the height of the waterfall is too high, the water will splash out and you pond will splash itself dry overnight.