I use apple cider vinegar in my dog and chicken water. It don't take much, about three table spoons per gallon. There are no side effects or over going and the benefits are many. It won't allow green stuff to grow in the waterers and it kills viruses plus it cuts gut fat and has lots of minerals in it. Works well for me.
I do the same thing with apple cider vinegar. I've swapped to plastic buckets instead of the rubber ones. I use the molasses tubs UglyDog mentioned since I'm away for work for two weeks at a time. This way my wife only has to feed when I'm gone. When I get back home I scrub the water tubs out with a scrub brush, put a little apple cider vinegar in them and fill with water. I repeat this a week later and then the day before I go back to work. Tubs are pretty algae free but i scrub them with a hard bristled scrub brush anyway. Some of the dogs like to cool themselves in the tubs by putting their front feet in them and standing in the water for a bit too.
