Giardia is a nasty and common parasite and, I think, requires a special test. It's easy to treat but doesn't go away very well on it's own.
Also, some dogs react badly to sudden and frequent diet changes. That's why the vet advised sticking with one for a while. When you do switch, do it gradually by slowly increasing the portion of the new food.
We had chronic soft (though not runny) stools with our little mutt. I finally switched him to a fresh, refrigerated food and his stool firmed up and his eye boogers stopped. But there is no one food that's going to work for all dogs.
Good luck.
Also, some dogs react badly to sudden and frequent diet changes. That's why the vet advised sticking with one for a while. When you do switch, do it gradually by slowly increasing the portion of the new food.
We had chronic soft (though not runny) stools with our little mutt. I finally switched him to a fresh, refrigerated food and his stool firmed up and his eye boogers stopped. But there is no one food that's going to work for all dogs.
Good luck.