Start by preparing the fish fillets. Rinse them under cold water and pat them dry with paper towels. Cut them into evenly sized pieces and set aside.
In a large mixing bowl, combine the all-purpose flour, baking powder, and salt. Whisk together until well combined.
Gradually pour in the cold sparkling water while whisking the flour mixture. Continue whisking until the batter is smooth and lump-free. The consistency should be similar to a thick pancake batter. If it's too thick, add a little more sparkling water. Set aside for a few minutes.
In a deep frying pan or pot, heat vegetable oil to a depth of about 2-3 cm (1 inch). Heat the oil over medium-high heat until it reaches a temperature of approximately 180 degrees Celsius (350 degrees Fahrenheit).
Dip each piece of fish into the batter, ensuring it is completely coated. Allow any excess batter to drip off before carefully placing the battered fish into the hot oil. Fry the fish in batches to avoid overcrowding the pan.
Cook the fish for about 3-4 minutes on each side or until the batter turns golden brown and crispy. Use a slotted spoon or tongs to carefully flip the fish halfway through cooking.
Once cooked, remove the fish from the oil and transfer it to a paper towel-lined plate to drain off any excess oil.
Serve the Battered Fish hot with lemon wedges on the side for squeezing over the fish. It pairs well with tartar sauce, coleslaw, or chips (fries).
