In a large heavy-bottomed pot, heat the 8 cups of vegetable oil over medium heat. Use a thermometer to check that it reaches 350°F (175°C).
In a mixing bowl, combine the 2 cups of flour, 1/4 cup of cornstarch, 2 teaspoons of sugar, 2 teaspoons of salt, 1/2 teaspoon of baking powder, 1/2 teaspoon of baking soda, 1/2 teaspoon of onion salt, 1/2 teaspoon of paprika, and 1/4 teaspoon of ground black pepper. Mix well.
Gradually pour in the 16 ounces of club soda to the dry mixture, stirring gently until just combined. The batter should be slightly lumpy.
Pat the 2 pounds of cod pieces dry with paper towels. Dip each piece into the batter, ensuring it is fully coated.
Once the oil is hot, carefully lower the battered fish into the oil using tongs. Fry in batches, making sure not to overcrowd the pot. Cook for 4-5 minutes, or until the fish is golden brown and cooked through.
Using a slotted spoon, remove the fish from the oil and place on paper towels to drain excess oil. Repeat until all fish pieces are cooked.