Find fresh or frozen wakame seaweed at Asian markets. You can also use fresh or frozen premixed seaweed salad. The tuna and avocado make this dish a potassium powerhouse. Serve atop brown rice, if you like.
Combine first 3 ingredients in a small bowl. Combine mango and remaining 6 ingredients in a separate medium bowl; toss gently to coat. Place about 3/4 cup mango mixture in each of 4 shallow bowls. Top each serving with about 1/2 cup tuna mixture.