With german sheps you have to be careful because their list of gastro intestinal genetic diseases is almost the longest of any breed. You'd have to do a full blood work up to be sure what the problem is, if it's not helped by changing foods. Usually the first stool of the day is firm and then every one after that is soft if there is a problem. (Makes sense since the colon works all night on the first stool and then a coupld hours on the subsequent ones)
I do notice that your food has a lot of wheat in it. That could be a cause too. Wheat is a common food allergen in doggies. I would give the trout and potato a try.