There's a few tricks you can use to keep it cleaned.
I gotta ask ? why are you filtering a pond for ducks? Is there a run off of some kind causing algae?
The ducks around our office usually head for the muddy ponds anyway.
Using a barrel , you can fill it with kiddie sand 2/3rds the way up.
Plump the incoming water to drop on top of the sand, but not from the top, hang a screen door type mesh 1 inch below where the water comes in to catch leaves and stuff.
Put outlet about 3 inches up from bottom as return AND put it 180 degrees from where the inlet is.
This is what we did for a neighbor who used rain run off from his shed to water his garden. Had the water tested, and was nice and clean.
IF there's an algae problem, that's different , but will still work , activated charcoal with help with nasties in the water BUT can be pricey.
Again why are you filtering the pond? seems a bit weird for ducks that's all.
Best of Luck
Bruce S