You need a high capacity sand/bio filtration system to mitigate the pond problem. Pond bottom filtration ALWAYS suffers from these problems. Think one the size of a 55 gallon drum sitting next to the pond and dumping back into it. Your pond is probably not big enough for the number of ducks you have.

There's a couple ways to get around this. We should talk so I can blather from memory. I'll also call a couple of people I know and get some advice. Email me a number keaerisdottir AT gmail DOT com.