First of all if your pet has chronic diarrhea, please take him to the vet for an overall physical. Many conditions can cause this and you need to know what is going on.
If everything has been ruled out like in my case with Snowball, then you might want to try some of these remedies. Snowball gets the runs every so often and they last for weeks. He is acting fine, no weight loss or vomiting or dehydration. I have tried all sorts of things to help stop it with no results, It usually stops on it's own. But this time I hit pay dirt!
I discovered in a people remedy book that apples, let to turn brown will stop diarrhea in humans. Plus bran which helps to loosen stools - also works to firm them. I added a little bran and chopped brown apples to Snowball's food twice a day.
To boost things along I also added a few drops of Crab apple from my Bach Flower Remedy collection. This I put in his water, and on his food. The trick is to get it in him at least four times a day for five days. If it works-- it works.
Low and behold, his stool firmed up and he has been fine. I tapered off the apple and bran and still good. I left the Crab apple in the water for the full five days. So far so good. If he relapses, I will just go back to the remedies - a brown apples, bran and crab apple again. Hope this helps your pet too!