Allska,
I don't profess to have vast knowledge about ducks and what to do with their manure but, I think it's a conservative guesstimate that a duck, (chicken, or goose) can input a pound of food and output 27 lbs of manure...Just my one-eyed math from raising a few. (foul, that is)
I suspect that a 5x5 pool is not going to be close to big enough for what you'd like to do. Nonetheless, best of luck with it and keep us posted...I for one would love to have 20 gallons of poopy duck water for my flower beds!