Nutri-Drench is a staple. Definitely always have it on hand. Not something you need to add to water daily, but definitely keep it in the first aid kit.
For prevention, immune boosting is the best thing or that. I like to add oregano, ginger, sometimes garlic, and what other herbs I have available to my poultry fermented feed. If your looking for probiotics, than fermenting a whole grain feed is an easy way to get that. I think herbs are fantastic, along with vegetables and fruits. All vegetables/fruits contain antioxidants, which I believe are vital for happy laying hens. They reduce free radicals in the body, and prevent premature aging, a big thing for laying hens, cancer, and tons of other diseases.
For prevention, I also give my chickens wellness blends on my Etsy shop if I have extra's, or blends that are getting old.