In a large bowl, combine the diced fish with the fresh lime juice and lemon juice. Make sure the fish is completely submerged in the juice.
Cover the bowl with plastic wrap and refrigerate for at least 2-3 hours, or until the fish is opaque and fully cooked from the acidity of the citrus juices.
While the fish is marinating, chop the red onion, tomato, cilantro, cucumber, bell pepper, and optional jalapeño.
In a separate bowl, mix these vegetables together.
After the fish has marinated and is cooked, drain off excess citrus juice, leaving a small amount for flavor.
Gently fold the chopped vegetables and cilantro into the marinated fish.
Add diced avocado and season with salt and pepper to taste. Gently mix to combine.
Serve the ceviche chilled, with tortilla chips or plantain chips on the side.
Enjoy your meal!