In a large bowl, combine the fish cubes with the lime juice and lemon juice.
Cover the bowl and refrigerate for about 2-3 hours, or until the fish is opaque and firm. The citrus juice will "cook" the fish.
While the fish is marinating, mix together the chopped red onion, green bell pepper, red bell pepper, minced garlic, and chopped jalapeños (if using) in a separate bowl.
Once the fish is ready, drain off excess citrus juice, leaving just a small amount to mix with the vegetables.
Add the prepared vegetables to the fish mixture and stir to combine.
Stir in the chopped cilantro, salt, and black pepper. Mix well.
Garnish with diced avocado, if desired.
Enjoy your meal!