Scoop the fish into a large nonreactive bowl (stainless steel or glass work best) and add the citrus juices and habanero chile. Fish should be completely submerged in juice—add more juice if the fish does not float freely. Allow the fish to “cook” in the citrus juices to your desired doneness—30 minutes to an hour for medium-rare, 3 to 4 hours for fully cooked. For a dryer ceviche, drain off and discard most of the juices from the fish. For a wetter (similar to a seafood cocktail) ceviche, leave as much of the marinating juice as you like.
Add the diced mango, avocado and red onion to the drained fish. Gently stir, taking care to not bruise or smash the avocado. Season with salt (usually about a scant teaspoon) and sugar, if you like it a little sweeter. Garnish with chopped cilantro and a few picked cilantro leaves. Serve with tortilla chips as an appetizer or spooned into rocks or martini glasses as a plated first course.