In a large bowl, combine the lime juice and lemon juice.
Add the fish pieces and mix well. Let the fish marinate in the citrus juices for about 1-2 hours in the refrigerator. This will "cook" the fish.
While the fish is marinating, chop the red onion, tomatoes, cucumber, and cilantro. If using, finely chop the chili pepper.
After the fish has marinated, drain the excess citrus juice from the bowl.
Add the chopped onion, tomatoes, cucumber, cilantro, and chili pepper (if using) to the fish. Gently mix everything together.
Season the mixture with salt and black pepper to taste. Adjust seasoning if necessary.
Enjoy your meal!