Preheat your oil to 350°F in a deep fryer or medium-sized saucepan.
In a medium bowl, mix the flour, cornstarch, baking powder, salt, and cayenne pepper. Whisk in the water until the batter becomes slightly foamy.
Pat the fish dry with a paper towel to remove any excess moisture. Dip each fish fillet into the batter, ensuring they are fully coated.
Deep-fry the fish fillets for 3 to 4 minutes until they turn golden brown. If frying multiple pieces at once, be sure to separate them to avoid sticking.