Preheat your oven to 425째F (220째C).
Place the potato slices in a large bowl of cold water and soak for at least 30 minutes to remove excess starch.
Drain and pat the potatoes dry with a towel.
Toss the potato slices with vegetable oil and salt.
Arrange them in a single layer on a baking sheet and bake for 35-40 minutes, flipping halfway through, until golden brown and crispy.
In a large bowl, combine the flour, cornstarch, baking powder, and salt.
Gradually whisk in the cold sparkling water until you have a smooth batter. It should be thick enough to coat the fish but not too runny.
Heat about 2 inches of vegetable oil in a deep fryer or large heavy-bottomed pot to 350째F (175째C).
Dip each fish fillet into the batter, allowing excess to drip off.
Carefully lower the battered fish into the hot oil and fry for 4-5 minutes or until golden brown and cooked through. Use a slotted spoon to remove the fish and drain on paper towels.
Enjoy your meal!