Chop you mango and avocado into small teaspoon size pieces then put into a dish. Add your red onion, jalapeño, orange juice, lime juice, sugar and salt then mix. Cover and refrigerate for about 30 minutes.
Take your ahi tuna and dice into teaspoon size pieces and set on the cutting board.
Remove your mango and avocado mixture from the refrigerator, mix with a spoon then drain some of the liquid. Add your ahi tuna and serve immediately with fresh tortilla chips.