If algae is part of your problem, barely extract does wonders.
If uneaten food and vegetation is a problem then we found crayfish do a good job at eating stuff that floats to the bottom. Next year we are going to try duckweed as plant cover since you can feed it to quail. Snail may also be an...
I don’t have ducks so I’m curious OP, is the issue algae and decaying material or duck poop? We have a turtle and after trial & error (and lots of wasted money) we created a pond environment via pool with minimal gunk cleaning.