Dog Food and runny poop?

I switched my dogs to the Canidae all stages food about 2 weeks ago from Purina One lamb and rice. Well, about the time that I switch the food, my dane started have runny poop. I thought that it was because he was on meds for coughing and just the new food. Well, he still has the runny poop but has been off the meds for about a week. I brought a sample to my vet and nothing was found. Could the food be too much for him? Any suggestions?