Whisk lime juice, cumin, paprika, garlic, salt, and pepper together in a large glass or ceramic bowl. Add tilapia and toss to evenly coat. Cover the bowl and marinate in the refrigerator for least 20 minutes, up to 24 hours.

Heat oil in a large pot over medium-high heat. Add onions; cook and stir 1 to 2 minutes. Reduce heat to medium. Add bell peppers, tilapia, and diced tomatoes in succeeding layers; pour in coconut milk. Cover the pot; simmer 15 minutes, stirring occasionally. Stir in cilantro; continue cooking until tilapia completely cooked through, 5 to 10 minutes more.