How many ducklings? What size was their pen? The way these guys go I simply can't imagine anything would help but close to home shavings are cheap and if I want to drive 30 miles they're fee and they even let you use their shovel. "fertilized" shavings are going out around the fruit trees.
I only had four in a kiddie pool, so not too many, but I only had to clean it once a day with four.