In a large bowl, combine the fish cubes with lime or lemon juice. Ensure the fish is fully covered by the juice.
Cover and refrigerate for at least 2 hours, or until the fish is opaque and "cooked" by the citrus juice.
Drain the fish from the marinade and gently pat dry with paper towels.
In a large bowl, combine the coconut milk, chopped onion, minced garlic, red bell pepper, green bell pepper, cucumber, tomato, and chili peppers (if using).
Add the marinated fish to the bowl and mix gently to combine with the other ingredients.
Season with salt and black pepper to taste.
Garnish with fresh cilantro or parsley.
Enjoy your meal!