Well, I've used a hydrocyclone before for something similar in an industrial setting:
I've never thought what I'd use on a duck pond though.
Any sort of cloth filter, sand filter etc is going to block pretty quickly, and then have to be cleaned out like you say (nice!!). The advantage of something like the hydrocyclone is that it operates automatically to blows the sludge clear, usually on a timed cycle.
How deep are your ducks' pockets?