May sound silly, but....does your dog lick it's anus alot? If so, you could be smelling anal gland fluid off of it's breath. Anal gland fluid can smell fishy or metallic. If your dog IS licking it's bum alot, consider checking with your veterinarian as this could be a sign of an impacted anal gland or underlying allergies. If this isn't the case, check to see if the food you are giving contains alot of fish or Omega fatty acids (usually fish oil).
OMG my dog has fishy breath too! I don't know what's wrong with him people say it's his diet but we've tried every single food ever sold
Yes that is normal. They make doggie mints which you can buy from basically any pet store.