In a large bowl, combine the diced fish with lime juice and lemon juice. Ensure the fish is fully covered with the citrus juices.
Cover the bowl and refrigerate for about 2 hours, or until the fish is opaque and "cooked" by the citrus juice.
While the fish is marinating, prepare the red onion, cilantro, tomato, cucumber, and peppers.
After the fish is done marinating, drain off any excess liquid.
Gently fold in the chopped red onion, cilantro, tomato, cucumber, and peppers. Mix well.
Season with salt and pepper to taste. Gently fold in the diced avocado if using.
Enjoy your meal!