Peel the potatoes and cut them into thick chips. Rinse in cold water and pat dry with paper towels.
Heat oil in a deep fryer or large deep saucepan to 180°C (360°F). You need enough oil to completely submerge the chips and fish.
Blanch the chips in the hot oil for about 3-5 minutes until they are partially cooked but still pale. Remove and drain on paper towels.
In a bowl, mix the flour, baking powder, and a pinch of salt. Gradually add the sparkling water, whisking until smooth. This is your batter.
Pat the fish fillets dry and season with salt and pepper. Lightly dust with cornstarch to help the batter stick.
Increase the temperature of the oil to 190°C (375°F).
Dip each fish fillet into the batter, letting the excess drip off, and then carefully lower into the hot oil. Fry for about 4-5 minutes until golden brown and crispy. Do not overcrowd the fryer.
Remove the fish and drain on paper towels.
Return the chips to the fryer and cook for an additional 2-3 minutes until golden and crispy. Drain well.
Season the chips with salt to taste.
Serve the fish and chips hot, with lemon wedges and tartar sauce on the side if desired.