Because dogs are smaller than humans, and because there are no recommendations for how much tuna a pup can eat safely, it's best not to feed it to your dog at all. Tuna is not toxic to dogs, and a tiny amount will not cause mercury poisoning.
How much tuna can a dog eat in a week?
Start by introducing a small portion and monitor your dog's reaction. If there's an allergy or another health issue, it's likely to become evident within 24 hours. Large breed dogs can eat one can of tuna over the course of one week, but not every week. Small breeds can safely consume about half a can over one week.