Combine flour, cornstarch, baking soda, baking powder, Old Bay, salt, and pepper. Add beer and stir. Let rest for 10 minutes. Dredge fish through mixture.
Preheat a deep-fryer or a deep pot filled halfway with oil to 350 degrees F. Gently immerse the fish in the oil, making sure it is fully submerged and deep-fry until it is golden brown and cooked through, about 4 minutes, turning to ensure even browning.