Enlarging pond, waterfall, pump size question

Question
HI, This might be a little lengthy. But here it goes.  I have a KoiPond installed with a 40 mil liner approx 300 gallon. This pond has a hand constructed creek leading to it, with liner of course, approx. 10 feet long. At the beginning of this creek is a small 1.5 x 1.5 pond. From the small pond the water flows down the creek over several natural very shallow waterfalls into finally the 300 gallon pond, which holds my fish. My Koi have outgrown this 300 gallon pond, and I am planning to add a 3000 gallon pond right below the 300 gallon, with a short creek and shallow waterfall, following the natural contour and downhill slope of the area.  My question is, how large a pump shall I get. The approx length from beginning of creek to the 300 gallon pond and the subsequent 3000 gallon pond will be approx 18 feet.  Hope this all makes sense.  

Answer
Hi,
I would say you want to be moving about 25% to 50% of your water every hour, so you will need a pump that can move that amount and if it has to pump a lot higher up to the top (head) you will have to take that into consideration , so a pump that will pump 1500 gall an hour should do the trick.
George.