Whisk the flour, cornstarch, baking powder, baking soda, salt, garlic powder, dried oregano, and red pepper flakes in a medium-sized bowl until well combined. Add half of the water and mix well. Add more water as needed to adjust the consistency. Set it aside.
Season the fish with salt and pepper on all sides.
Pour the vegetable oil into a medium-sized pot and set it over medium-high heat. Let it heat for 3 to 4 minutes. If it begins to smoke, reduce the heat slightly.
Meanwhile, add the mahi mahi pieces to the batter. Stir them around until they're entirely coated.
Take one battered fish and dip the tip into the oil; if it sizzles, the oil is ready to fry the fish. Gently lower a few battered mahi-mahi pieces into the hot oil. Do not crowd the pan. Fry the fish for 30 seconds on each side or up to 1 minute until golden brown.
Remove fish from the oil and set them on a paper towel-lined plate or cooling rack. Let the oil reheat for 30 seconds before frying the next batch.
In the meantime, whisk together the mayonnaise and chipotle until you get a smooth mixture. Set aside.
Place the tortillas on a griddle or comal set over medium-low heat. Let them heat for 1 to 2 minutes on each side or until warm. Remove them from the heat.
Place the tortillas on a serving plate. Also, place the fried fish, shredded cabbage, and carrots on separate plates.
Let everyone assemble their tacos with a tortilla, fish, and favorite toppings.
Top with chipotle mayo and hot sauce to taste.
Garnish with chopped cilantro and serve.