Combine pineapple juice, lime juice, salt, oregano, garlic and pepper. Mix well. Pour this mixture over the Mahi Mahi and let marinate for 24 hours. Remove the fish from the marinade and cut into 1/4-inch cubes. Grill or saute the fish. Place the fish in the tortillas and garnish with cilantro, onion and pineapple salsa.
PINEAPPLE TOMATO SALSA:
Add the pineapple to a blender. Combine the tomatoes and tomatillo in a bowl and mix well. Add half of the tomatoes to the blender. Add the lime juice, garlic, ginger and salt and pepper to the blender. Blend until smooth. Pour into a bowl and add the remaining tomatoes, cilantro, mint and jalapeno and mix well. Season to taste with salt and pepper.