Start by heating your oil in a deep fryer or large pot to 350°F. Use enough oil to fully submerge the fish and fries.
Rinse and pat the cod fillets dry. Lightly season them with salt and pepper.
In a large bowl, whisk together the flour, baking powder, salt, and pepper. Slowly add the sparkling water while whisking until a smooth batter forms.
Dip each cod fillet into the batter, coating evenly. Let the excess drip off.
Gently lower the battered cod into the hot oil. Fry in batches for 4–5 minutes or until golden and crispy. Don’t overcrowd the pot.
Transfer cooked fish to a wire rack or paper towel–lined plate to drain.
Add the potato fries to the hot oil and fry until golden brown, about 6–8 minutes.
Sprinkle the fries with salt immediately after frying.
Serve hot with lemon wedges and your favorite dipping sauce—tartar is classic, but I won’t stop you from using ketchup.