We use Apple Cider Vinegar in our ducks water, both drinking AND their pool.
ACV is full of vitamins, minerals and trace elements. It helps to lower the pH level in the stomach, helping digestion and making it a less friendly for harmful pathogens. ACV detoxifies the blood and helps remove mucous from within the body, this is particularly useful since chickens/ducks are particularly prone to respiratory problems and ACV can be of benefit in helping birds to clear their airways.
Due to the acid content of ACV, it is an antiseptic. As well as killing germs, it is also a mild antibiotic as well, (that is, it contains bacteria that destroy infectious organisms).
It also changes the PH in the pool water, and keeps algae from growing like crazy. I LOVE it.
FYI though, you have to get the raw, unfiltered kind, (looks like it has floaties lol) You can get it at health stores, and various other places.
we put a cap full in the drinking water, and just eye it and splash it in to their pool, but I am sure you can find the right dilution amount online too.