For the tempura batter: Sift together the flour, cornstarch and salt in an oversized bowl (you will also dredge the fish in this). Add the egg yolks and half of the sparkling water and stir until it forms a paste. Add the remaining sparkling water and stir to combine. Place in the refrigerator until needed, ideally allowing it to rest for 1 hour.
For the fried fish: In a deep saute pan or small wok, heat the oil over medium-high heat. It is ready when you drop in a bit of batter and it sizzles and floats almost immediately. If you have a candy or deep-frying thermometer, heat the oil to 375 degrees F.
While the oil is heating, season the fish lightly with salt and allow it to come up to room temperature.
Working in 3 batches, dredge the fish in the tempura batter, then place in the hot oil. Fry on one side without disturbing the fish for about 2 minutes, then flip and fry for 1 more minute or until the batter is golden. Remove to a wire rack or paper towel and season with some salt and a squeeze of lime. Fry the rest of the fish and then serve warm with tortillas and taco fixings.