Mother Earth News recommends giving your chicken a potassium permanganate solution to drink. Dissolve 1 tablespoon of the chemical in 1 quart of warm water. Feed each chicken 1 tablespoon of the concentrated solution diluted in 1 cup of warm water. In a case of severe diarrhea you may need a stronger solution, one that is potent enough to turn your finger slightly brown when you dip it in the liquid. Be sure to not keep the potassium permanganate mixture in a metal container.
Another remedy is Epsom salt. Add one-half tablespoon of Epsom salt to the affected chicken's feed. You can also feed your diarrhea-afflicted chicken wheat bran moistened with sour milk or buttermilk.
If diarrhea keeps occurring despite all your attempts to alleviate it, take your chicken to the vet. Your bird could have a serious illness that requires medical treatment.