In a medium bowl, combine tomato sauce, mango nectar, ketchup, brown sugar, vinegar, molasses, and garlic powder. Whisk thoroughly, and then stir in diced tomatoes and mango.
Place fish and sauce in a container and toss to coat. Cover and let it marinate in the refrigerator for 30 minutes.
Bring a skillet sprayed with nonstick spray to medium heat on the stove. Add fish and marinade. Once marinade begins to simmer, cover and cook until the fish is tender and cooked through, about 5 minutes.
Plate the tilapia and top with any excess marinade from the skillet. Garnish with cilantro and enjoy!