There is a link to a thread on housing, runs, etc., on the duck forum sticky topics post that has a lot of discussion of ponds and filtration that may be helpful to you. The amount of waste our four ducks produce in a kiddie pool in a day is substantial. We used to keep large aquariums for fish, but none of those large waterfall type filters would have had a chance in the face of four ducks. I’ve heard of people using multistage bio filtration systems, but the multiple tubs or barrels take up a lot of space. Most ornamental pond filters aren’t up to the task long term.
But if it can be done and done on a budget, this is the website to find out how they did it. We want a small, inground pond, but the cost of a workable filtration system isn’t in our current budget.