Ideally all animals have their teeth brushed every day, but this is not an ideal world. Just because you don't see a lot of overt and advanced dental disease does not mean that dental problems don't exist. Plaque is invisible and forms quickly after brushing. Our own breath would be just as foul if we did not pay diligent attention to our own oral care.
Now consider that dogs lick themselves (yes, even back there), and imagine what is being populated in the canine mouth. My suggestion is to brush your pet's teeth every day with a non-fluoride paste: dogs do not spit out tooth paste, and fluoride is toxic if swallowed often enough.
There are many new dental hygiene products available, consult your veterinarian. Of course, I am assuming that there is no gastrointestinal problem involved as your statement declared. Additionally, I would ask your veterinarian to check your dog's anal sacs, and express/drain them if they are full--this is a source of many fishy odors.
Dr. Van Lienden
Dr. Raymond Van Lienden DVM
The Animal Clinic of Clifton
12702 Chapel Road, Clifton
Virginia, U.S.A. 20124