Ha ha! You are doing NOTHING wrong. I clean pools daily too. They are disgusting within five minutes.
Number of ducks makes a big difference. With them divided into breeding groups, they only get free range time in shifts. So some days there are only four or five ducks using the "communal" pool in the middle of the yard, and I'm amazed at how undisgusting it is at the end of the day. Dirty, yes. But not completely revolting, and I can sometimes almost see an inch or two into the water through the muck.
Sometimes, I deliberately let one of the pools get really mucky. Then I dump it into an area that I want prepped for planting. The ducks gladly do my digging and tilling for me, and at the end of the day the area is a solid grass-free mud puddle that in a few days is ready to put a tree or flower in (could probably prepare seed planting beds this way too.... hmmm, what would happen if I did that on top of a straw bale & then planted lettuce seeds??). I also dump muck water out around the base of fruit trees, and it does amazing things for their growth. Aerated compost tea.