In a medium bowl, stir together flour, baking powder, baking soda, and cornstarch.
In a large bowl, beat together eggs and milk. Mix in beer. Stir in flour mixture. Season with salt, black pepper, and garlic powder.
In an electric deep fryer or a heavy saucepan, heat oil to 375 degrees F (190 degrees C).
Coat fish in batter, and submerge in hot oil. Fry until golden brown, about 4 to 5 minutes. Serve.