UPDATE
After the crying, you start thinking.
First, I walked everywhere in my yard, even if the ducks had not been there. I found some plastic that had blown off of a wood pile, probably in the spring, and had been left on the ground. Wonderful. Yes it had pockets of water on it and leaves and gypsy moth droppings and was pretty nasty. With all of the rain and then heat, gee, ya think? Great, way to go Deb. It very well could be botulism.
Second, locked ducks and geese out of their pen and added 8 cups of bleach to their 1100 gal. pool. Not enough to harm them but enough to stop anything more from growing. Ran the filter to mix it in then let it set for 5 hours. Drained and replaced half the water, added more charcoal to the filter and then ran it for 3 more hours. I really wish I knew if you can use the same chemicals in their pool as I do in mine. Life would be so much easier.
Third, when my husband got home from work, we started rototilling their pen. Heavy barn lime application and deep tilling. Added all of the sand I had. [the ground here is heavy on the clay and it quickly turns into a mess] Sanitized and cleaned everything having to do with their feed and water.
Have I missed anything?