Peel the potatoes and cut them into thick, even strips.
Rinse the potato strips in cold water, then soak them for 20–30 minutes to remove excess starch.
Drain and dry the potatoes thoroughly with a clean kitchen towel.
Heat vegetable oil to about 160°C (320°F). Fry the potatoes in batches for 4–5 minutes until softened but not browned. Remove and drain.
Pat the fish fillets dry and season both sides with salt and black pepper.
Lightly coat each fish fillet with ½ cup flour and shake off any excess.
In a bowl, mix 1 cup flour, cornstarch, baking powder, salt, and black pepper.
Gradually whisk in the cold sparkling water until you have a smooth, slightly thick batter.
Dip each floured fish fillet into the batter, making sure it is evenly coated.
Heat the oil to about 180°C (350°F). Carefully place the battered fish into the hot oil and fry for 4–6 minutes, turning once, until golden and crisp and the fish is cooked through.
Remove the fish and drain on a wire rack or paper towels.
Increase the oil temperature to about 180–190°C (350–375°F) and fry the potatoes again for 2–4 minutes until golden and crispy.
Season the hot chips with a little salt.
Serve the crispy fish and chips with lemon wedges and tartar sauce.