Heat vegetable oil in a deep fryer or large pot to 350°F (175°C).
In a large bowl, whisk together flour, cornstarch, sugar, salt, baking powder, baking soda, onion salt, paprika, and black pepper.
Gradually add club soda, whisking until the batter is smooth and lump-free.
Pat dry the fish fillets, then dip them into the batter, coating evenly.
Carefully place the battered fish into the hot oil and fry for 3-4 minutes per side, until golden and crispy.
Remove and drain on a wire rack or paper towels.
Repeat with remaining fillets, ensuring the oil stays at the right temperature.
Serve hot with tartar sauce, malt vinegar, or your favorite dipping sauce.