I dealt with diarrhea recently with my horse. His diarrhea ranged from "cow patties" to projectile. I had to clean his butt and back legs daily, very tough during the winter!
He maintained his weight, ate normal, drank normal, just had diarrhea. Everything checked out normal with the vet. I tried BioSponge and it did not help at all. I stopped feeding him alfalfa grass mix hay thinking the alf was causing the loose stools- nothing changed. I tried every probiotic under the sun to no avail. My vet recommended slippery elm, which finally dried him up. Slippery elm turns from a powder to a gel during digestion and coats the hind gut. All winter he got slippery elm twice a day, along with yea-sacc once a day. I recently started giving him slippery elm once a day and he has continued to have normal stools. He does have a bad day once in awhile, but the difference is huge.
It is such a frustrating problem! I hope your horse starts doing better soon.
14 year old Quarter Horse gelding "some horses come into our lives and quickly go, others stay awhile, make hoof prints in ours hearts and we are never, ever the same"