Pond pumps and underlayment

Question:
Pond Pumps: GPH and Water Circulation?
Why does a pump have to circulate 1/2 to the entire volume of water in one hour?
What pond liner, underlayment, and pump would be suitable for a pond 20 X 20 feet that is 3 feet deep?


 Answer:
Use 40 mil butyl rubber for a liner. You can use commercial underlayment or roofing felt. Old carpet can rot and start to stink as time passes.
To find liner size, measure the bottom, both sides and add 6'. I would get a 3500 gph pump so water will properly circulate. Circulating water adds O2 to the water, so the fish can breathe.