Fishy breath in dogs is often a symptom of causes that include gum and tooth problems, according to Petco. Issues like rotting teeth and periodontal disease cause foul-smelling breath as a direct result of infection and bacteria in the mouth.
Fishy-smelling dog breath can also be the result of the dog licking its anal area when the anal glands need to be expressed. Typically, blocked anal glands can cause a foul, fishy smell. In certain cases, foul-smelling breath can be a symptom of diabetes or kidney disease, according to VetInfo. However, foul-smelling breath from disease is typically accompanied by other symptoms.