Marinate the Fish: Place the diced fish in a glass bowl and pour lime juice over it. Ensure the fish is fully submerged. Cover and refrigerate for 15-20 minutes.
Prepare Mix-Ins: While the fish marinates, dice the mango, avocado, and red onion. Chop the jalapeño and cilantro.
Combine: Drain most of the lime juice from the fish, leaving about 2 tablespoons. Gently fold in the mango, avocado, red onion, jalapeño, and cilantro. Season with salt and pepper.
Serve: Serve immediately with tortilla chips, tostadas, or lettuce cups.