In the case of suspected food allergies, it will take at least several weeks for your pet to recover from an allergy, so when testing by means of sampling or removing one type of food from their diet, be sure you permit sufficient time for your pet to recover and respond to the change. My former vet permitted one day for each sampling, which in hindsight was ludicrous as not only did it serve no purpose in fact finding, but it actually upset her digestive system even more to have so much alteration to her diet so quickly. Become educated.
In the case of diarrhea, most vets will suggest Kaopectate or Pepto Bismol, both of which are dangerous. In the case of Kaopectate, it has been reformulated to contain bismuth subsalicylate in addition to the old ingredients which were attapulgite and alumina clay. Some larger dogs can handle this compound, but if too large a dose is given especially to small dogs and cats, it can be toxic. Surprisingly, your vet may or may not know this . . . mine didn't.