Chickens seem to prefer drinking the nastiest water they can find. It is our job to make sure they only have fresh clean water. From my own reading, experience, and talking with others, the best way to do this (unless you are supplying them directly from your own drinking water) is to add hydrogen peroxide to the water tank. Hydrogen Peroxide (H2O2) is a powerful oxidizer that kills (literally by burning) any organic matter in the water. Keep in mind that, ideally, by the time the water gets to the chickens, the hydrogen peroxide will have broken down into water by releasing the extra atom of Oxygen. Our objective is to have clean water – and only clean water – for the chickens. We aren’t trying to feed them hydrogen peroxide.
When using hydrogen peroxide, it is extremely important that it be properly diluted – contact with high concentrations can be quite dangerous, and if it is too low, it will be less than effective. When handling concentrated hydrogen peroxide, you need to be very careful – eye protection and rubber gloves would be a good idea.
The 35% food grade hydrogen peroxide can be found at most health food stores. It needs to be kept refrigerated and away from light. Again, read the warnings and handling instructions carefully.
The recommended concentration is 8 ounces of 35% food grade hydrogen peroxide per 1,000 gallons of water. For my 35 gallon tank, I use 1 1/2 teaspoons in 30 gallons of water (I don’t fill the tank all the way to the top). I have since switched to a metric graduated cylinder to measure, so that comes out to 7.097 ml of 35% food grade hydrogen peroxide.