Chill the club soda. Set a wire rack over a sheet pan. Pat fish dry and cut into 1-inch-wide strips.
Heat oil in a deep, heavy pot to 350°F over medium-high heat.
Whisk flour, cornstarch, baking powder, baking soda, salt, sugar, paprika, onion powder, garlic powder, and white pepper in a large bowl.
Pour in the cold club soda and stir just until a smooth, thin batter forms. Do not overmix.
Dip a few fish pieces into the batter, letting the excess drip back into the bowl.
Fry in batches for 3–4 minutes, turning as needed, until deep golden and crisp and fish flakes easily.
Drain on the rack and lightly salt. Repeat with remaining fish. Serve hot.