A low alarm should be good. It buzzes off everytime the water reaches the defined threshold. Did a quick search and found one by Honeywell - https://www.amazon.com/dp/B00BIBD4EY/?tag=cattletoday00-20
I could be the parasites or coccidio or altogether something different. And there's also a possibility that it has nothing to do with the feed. There could be some other issue ailing the remaining 10%. Are these 10% huddled together ?