I am a firm believer in eating everything in moderation. For every dietary plan that I supply to my clients, there will always be a mix of:
Nuts (whether whole or in LSA)
Restricting yourself to a particular food may have side effects because you may be restricting yourself from a particular nutrient. Many bodybuilders and fitness fanatics do this in a very unhealthy manner which can lead to long-term health complications. For example, eating only banana's as fruit. Whilst they are a great source of potassium, they are quite low in other nutrients and could easily contribute to a nutrient deficit.
Conversely, eating too much of a particular food could have side effects too. A common example is consuming too many carrots. A high intake of beta-carotene could actually make your skin turn orange.
In your case, a high intake of tuna could potentially have side effects. Tuna has a reputation for being relatively high in mercury relative to other foods. It's not dangerously high (otherwise it wouldn't sold in supermarkets), but a significant and regular intake could have a cumulative effect on mercury levels.
Whilst your body can metabolise mercury, it is a slow process. So, ingesting substantial amounts of mercury could lead to a mercury build-up over time.
As a first resort, I would suggest to see your doctor and to test for mercury levels if this is a concern of yours.
One course of action may be to avoid the seafood for the time being. Instead, consume meats. Mix up the meats - chicken breast, lean beef, turkey, low-fat ham etc. Don't eat chicken breast exclusively either, other meats contain minerals that are required for normal functioning. For example, turkey is rich in selenium, a good antioxidant and responsible for healthy skin, nails and hair. Beef is rich in iron, responsible for energy and normal functioning. A little seafood here and there may be healthy, but do not eat it exclusively.