A helpful Q&A from SmartPak for those with similar challenges:

I have a 26 year old TWH that has chronic diarrhea. He had vet work done last year, when it first started, with his blood work coming back normal, fecal exam okay and teeth are okay. No feed or hay change. Actually, his stool will be somewhat normal, not firm apples but has some substance to them, for most of the time then about once or twice a month it gets loose to the point of liquid. I had him on ProBalance during some of this time period but it did not seem to affect his stool. He is now on Stomach Smoother but still having the same results as with ProBalance. His coat is shiny, he eats all his grain and most of his coastal hay (his preferred hay for 23 years) and is still very active. His weight is pretty much staying the same the last year. - GB

Answer:

http://blog.smartpakequine.com/2012/07/what-to-do-about-horses-with...