Peel and de-vein the shrimp, leaving the tail shells on.
Slit the curled underside of each shrimp with a sharp knife and stretch the shrimp out straight. Pat them dry with paper towels.
Preheat an air fryer to 350 F.
In a shallow bowl, whisk together ½ cup of the flour with the cornstarch and salt.
In a separate bowl, beat the egg and seltzer together.
Pour the egg mixture into the flour mixture and whisk until just combined.
Prepare a second shallow bowl with the other ¾ cup of flour.
Dip each shrimp into the first bowl of wet batter, let them drip, and then dredge them in the second bowl of flour to seal the batter. Set the shrimp aside on a dish. Give the shrimp a second pass in the flour if they still look wet.
Place the shrimp in a single layer in the air fryer basket. Do this in two batches to avoid overcrowding them.
Air fry for 7-9 minutes until lightly browned and crispy.