In a large glass or ceramic bowl, combine the fish, shrimp, and conch (if using).
Pour the fresh lime juice and orange juice over the seafood, making sure it is fully submerged.
Cover the bowl with plastic wrap and refrigerate for at least 2-3 hours, or until the fish is opaque and "cooked" through.
While the seafood is marinating, dice the tomato, chop the red onion, cilantro, cucumber, bell pepper, and jalapeño peppers.
Once the seafood is marinated, drain off most of the marinade, leaving a little to keep the mixture moist.
Add the diced tomato, chopped red onion, cilantro, cucumber, bell pepper, and jalapeño peppers to the bowl.
Gently mix all the ingredients together until well combined.
Season with salt and pepper to taste.
If using, gently fold in the diced avocado just before serving.
Transfer the ceviche to a serving bowl.
Enjoy your meal!