Place the fish fillets in a container and pour the buttermilk over top.
Place the container in the refrigerator and marinate for 30 minutes.
Meanwhile, in a large shallow dish, mix together the panko crumbs, cornflake crumbs, breadcrumbs, lemon zest, cayenne pepper, Old Bay seasoning, garlic powder, salt, and pepper.
Remove the fillets from the fridge, allowing the excess buttermilk to drip off, and press each fillet into the crumb mixture on both sides.
Place the breaded fillets on a plate or baking sheet and return the breaded fish to the refrigerator for at least 30 minutes.
In a wide, high sided frying pan, add enough oil to fill the pan halfway with oil
*DO NOT OVERFILL*
Heat the oil over medium/high heat to 375-degrees.
*Note - if you don't have a thermometer, you can tell when the oil temperature is ready by sticking the handle of a wooden spoon into the hot oil. If lots of small bubbles emerge from the spoon (similar to champagne bubbles), the oil is ready.
Carefully slide a few breaded fillets into the hot oil, making sure not to overcrowd the pan.
Fry each fillet until golden brown on both sides (approximately 2 minutes per side).
Carefully remove the fried fish from the oil using a slotted spatula and place on a baking sheet that has been fitted with a rack.
Immediately season the hot fried fish with additional salt.
Continue the same process until all the fillets have been fried.
Keep the fried fillets warm and crunchy in a low 300-degree oven while frying the remaining fish, if needed.
Serve hot with lemon wedges and tartar sauce if desired.