I have tried 3 pumps so far for our duck pond and none of them seem to last very long, approximately a few months and then they quit working or decrease their output. We first bought a sump pump, then a sewage pump from Home Depot, then another sewage pump from Home Depot. The last pump was supposed to be able to pump 5700gph, it pumped great at first but now it can't seem to pump out as much water.
Our pond is approximately 18' x 21' and approximately 4000gallons. We have two large filters filled with lava rocks, that combined are about 400-500 gallons. The pump has to pump the water about 20 feet to the filters and up about 5 ft.
I'm curious if anyone else has a pond this size and what kind of pump you are using?