I noticed that my falls was running a little slower, so I pulled up the pump, and - sure enough - it was clogged with gunk. I hadn't checked it for months. And when the koi got into the lily section of the pond and tore up so much plant material, this added to the debris being sucked toward the pump's pre filter. Good thing it's on there.
Nelson's Water Gardens, our biggest local pond store, removes and throws away those foam pre filters. They don't filter well, and clog quickly and bog down the filter. They recommend a basket type pre filter, which is what I use. Works great.