Ducks are really messy at times the number of ducks you have verses the pond size is critical when choosing a filtration system. One thing to help is an aerator, on large scale can be made out of a simple sump pump connected to a waterhole with a wire mesh end to suck water through and a pvc pipe used to shoot water into the air like a fountain, to keep the water oxygenated. Germs and bacteria can form easily if its too small of a pond and would be unsanitary even for the ducks to drink from.
If you want to do it right, with little expense, and it really works a lot better....ask any old timer. Find some blue plastic barrels three if you can find them use them as your filtration system, using carbon / charcoal, sand, and gravel. Some would call it a home made septic system or giant fish tank filter system/ if you look online...Use the sump pump to fill the system with water from the pond and allow the water to flow back into the pond maybe with a home made waterfall. I would only do this if your creating a small pond for your ducks. If your going larger, you need to consider the ecosystem of the pond your building and what it will take to help keep it clean.
Hope this helps, good luck!
