Peel and cut the potatoes into thick chips and soak them in cold water.
Drain and dry the chips before frying.
Heat oil in a deep fryer or large pot to 180°C (350°F).
Mix flour, cornstarch, and baking powder; add cold sparkling water to create a batter.
Season fish fillets with salt and pepper, then dip into the batter.
Carefully lower the battered fish into hot oil and fry until golden brown.
Fry the chips in the same oil until crispy and golden.
Remove fish and chips, draining on paper towels.
Serve hot with tartar sauce and lemon wedges.